Celestial Coordinates Tutorial

this is a tutorial video on celestial coordinates specifically the local coordinates of altitude and azimuth as well as the celestial coordinates or the equatorial coordinates of right ascension and declination what we should really call celestial latitude and longitude but astronomers call them right ascension and declination i'm going to be uh going over these coordinate systems in the program stellarium so this is also a bit of an introduction on some of the features of the planetary uh planetarium program stellarium all right so uh first of all let's talk about those local coordinates altitude and azimuth um altitude is just the angle that something makes in the sky relative to the horizon uh altitude sounds like something uh you would measure on a plane altimeter and it would have units of meters it would be the height above the the ground or the sea level uh that's not what astronomers mean when they say altitude it's an angle so it's going to be measured in degrees i think we could measure it in radians or some other unit but degrees degrees arc minutes arc seconds and it's the angle that the object makes with the horizon so i there you see the sun kind of over in the east and if i actually click on it stellarium will give me information about its altitude and azimuth by default it actually gives you a lot more information but one of the things i've done here if you go into the configuration window for stellarium you can you can control how much information you see when you click on an object so if i click on that and or you hit f2 key and if i go to this information tab right here then selected object information i just want to see the name and its altitude and azimuth for right now so you see the altitude of the sun is 47 degrees 59 arc minutes and some mark seconds and when this turns over to zero the next degree marker goes up so it's at 48 degrees above the horizon right there now the horizon has an altitude of zero so when an object is rising or setting its altitude is zero and then the highest altitude you can have is directly overhead that's what astronomers call the zenith or zenith and that's an altitude of plus 90 degrees you could have negative altitude which would mean the object is below the horizon so you normally wouldn't be able to see it and that's perfectly fine and then at negative 90 degrees would be what we call the nadir nadir opposite the zenith so uh the reference plane for this local coordinate system of altitude and azimuth is the horizon um the other uh angle is the azimuth and azimuth is just where along the horizon is the point directly below or above below if it's a if the object has a positive altitude so that's it's a above the horizon and and then the point that would be above an object that was negative altitude below the horizon uh so you drop down to the point on the horizon uh below that up below where that object is like there's the sun right there i just drop straight down and then that intersects the horizon somewhere and now i can measure its azimuth azimuth is really just a compass heading compass bearing around the horizon from the reference point which is due north so due north is 0 degrees azimuth due east is 90 degrees azimuth due south is 180 degrees as azimuth and then due west is 270 degrees azimuth uh and then when you get around to north again it resets to 360. so you get the 358 359 0 degrees azimuth so if we look back here at the sun there's the sun right there i dropped straight down it's now 48 degrees above the horizon right there and it's azimuth is should be a little bit more than 90 degrees because it's a little bit south of due east and you see its azimuth is 101 degrees right there i actually have a plug-in in stellarium that lets me show the compass marks so if i turn that on you can see oh yeah there are the azimuth angles from zero to the north 90 to the east 180 to the south and so forth so that's altitude and azimuth um it's kind of like the coordinates you might put into a cannon if you were going to aim it somewhere you'd say okay elevation or altitude 45 degrees above the horizon you turn a big crank and it goes the gun goes up to a steeper steeper angle and you say heading 165 degrees and then you turn another crank and it turns and points in that direction and so those are the local coordinates there's an important reference line for the local system which is called the local meridian that is a line which goes from due south directly overhead through the zenith and then down to due north and intersects the horizon due north so let me turn that on if i go over here to sky and viewing options window or hit the f4 key one of the options is markings and you see here celestial sphere i can draw a meridian line so now there's a line right there kind of greenish that goes from 180 through the zenith up there and then if i turn around to the north it goes into the north horizon now technically there's an upper meridian and a lower meridian um but for most purposes it doesn't matter you can just think of the meridian as going from south to overhead to north and that's that's what's important now the meridian is a useful reference line because everything uh that rises and sets through the sky has to cross the meridian at some point um if i speed time up you can see that there's the sun going across the sky from east to west some point the sun sets over there and uh the earth is spinning way too fast there goes the sun again across the sky um you see i still am showing its altitude and azimuth there it's negative it's below the horizon there when it's positive it's above the horizon you see the altitude is increasing until it gets to 360 and then it flips around to zero that's true for all the stars as well let me slow this down a little bit here all right so there's the sun crossing the meridian that's going to be local solar noon that's when it's going to be at its highest possible altitude and of course its azimuth has to be exactly 180 degrees because the meridian everything on the meridian has an azimuth of 180 degrees if it's south of the zenith or zero degrees if it's north of the zenith let's see when the sun goes down again we'll look at a couple of different stars so let's watch the sun go down the sun's setting north of due west because it's summertime and there are the planets that are out tonight and then here are some bright stars let me just bring the playback back to normal speed here and there's the star vega right there now vega you can see has an azimuth of 75 degrees so it's kind of a basically east a little bit north of east at that point in time right there's nine degrees is due east let me go back to the uh the direction markers like so so there's due east right there and it's altitude's pretty high it's at 50 degrees and 29 arc minutes and as time passes its altitude gets higher and higher and if i speed time up again speed time off by 10 100 a thousand we can watch its altitude get higher and higher until it crosses the meridian so here at some point we'll pause when i get close all right right about there i didn't get it exactly but i'm close it's an azimuth 176 and 54 arc minutes increasing and there you see its altitude 87 degrees and 29 arc minutes oh it's very close to 90 degrees at this particular latitude in new haven vega almost passes through the zenith but not quite exactly right not quite exactly so it's still going up very gradually because i'm not quite at the meridian once it crosses the meridian this number is going to stop going up and start going down so uh astronomers generally want to observe an object when it's as close to the meridian as possible because we want to observe objects when they're at their highest possible altitude that is because when they're at the high altitude uh you're looking through less atmosphere if we look at an object that's down here low at low altitude close to the horizon we're looking through maybe two or three times as much atmosphere as when we look overhead and the atmosphere has the effect of dimming objects that's why the sun is so much dimmer when it's rising or setting it also will redden objects so just like the sun looks very red at sunrise and sunset uh because the atmosphere is preferentially scattering blue light so the sun is uh the light from the sun is being scattered to make the sky blue somewhere west of you when you see a nice red sunset the other effect that the atmosphere has is that the turbulence in the atmosphere will cause the stars to twinkle or shimmer if you like and that will blur out your view of the universe when you're looking through extra atmosphere i mean you'd love to have uh unlimited time on the hubble space telescope so you can not look through any atmosphere but here with a ground based telescope you have to minimize the atmosphere you look through so you want to look when it's as high in the sky as possible the highest possible altitude now you can see vegas crossed the meridian so the number the altitude here is slowly decreasing so um let me just to review that turn on the altitude and azimuth grid so if i go down here however to hit the z key stellarium turns on this altitude and azimuth grid so you can see these wedges that are coming up and converging at the zenith those are wedges of particular azimuth constant azimuth and then these circles that go up from the horizon to the zenith those are circles of constant altitude so that's 10 degrees right there 20 degrees 30 degrees and so forth all the way up to 90 degrees and then you can see the same with the um the azimuth markings here as the earth turns the stars will change their altitude and azimuth as they rise in the east cross the meridian reach a maximum altitude and then start setting in the west i have a nice uh kind of simulation of this as seen from outside the celestial sphere uh outside the dome of the sky now this is of course an impossible thing to do um but this is kind of a convenient model to have this is based on the university of nebraska astronomy simulations you can go to this website to see the the animations and simulations that they made for their astronomy classes at university of nebraska and they put these online they're very useful for teaching intra astronomy anyway this one is the azimuth altitude demonstrator and you see a little stick figure there in the middle of his horizon and you can see north south east west marked on the horizon you can always figure these out because by definition the direction the earth turns is east so this must be the direction the earth has to turn and then 90 degrees to the left of that is north 90 degrees to the right of that is south and so forth so you can see those reference points you can see the meridian right here going from south to overhead to north there's the zenith there's the nadir and you can just see uh the altitude 45 degrees uh up to close to 90 degrees down to 10 degrees or less that can be negative if it's below the horizon uh and then you can also see the changing azimuth right here right so if i drag the star around to the north the azimuth decreases like so and if i drag it back around to the south the azimuth increases if it were exactly on the meridian its azimuth would be exactly 180 degrees and so i can specify any point in the sky by using these two angles with zero altitude at the horizon and zero azimuth at the north horizon due north on the north horizon if we go to zero azimuth and zero altitude that should be exactly on the north horizon dial that in let's put zero in here in fact there we go so there's that a star that we're at that point would be zero altitude and zero azimuth all right um well let's move on to talk about celestial latitude and longitude uh this is a coordinate system which doesn't actually depend on your local horizon this is a system that's based on the celestial equator and the celestial north and south pole as well as the ecliptic the path the sun takes uh in the sky so uh celestial latitude and longitude or um in astronomy we call them declination for celestial latitude and right ascension for celestial longitude uh they're defined very much in analogy with terrestrial latitude and longitude uh so every point on the surface of the earth has a particular latitude and longitude and the latitude is measured from the equator around the curvature of the earth so the equator would be zero latitude the north pole would be plus 90 degrees north or just 90 degrees north the south pole would be negative 90 degrees or just 90 degrees south and then there's also a longitude which is the east-west coordinate now there's no good place to define zero degrees longitude it's completely uh arbitrary um but uh we have by convention chosen london or actually the greenwich observatory uh as the reference point as for zero degrees uh longitude so uh i can actually turn on the latitude and longitude system uh here in google earth if i go up here to view grid right there you can see the latitude and longitude for everything on the earth so zero degrees is the prime meridian which goes through greenwich observatory ah an observatory uh there must be an astronomical reason for choosing that point for zero degrees longitude uh there is uh maybe i'll talk about that in a separate video about time keeping and celestial navigation uh but that's just what we've chosen by convention and then zero degrees latitude is the earth's equator which is of course midway between the north and the south pole so in a sense the latitude and longitude grid is defined by the rotation of the earth because the earth rotates on its axis that defines the north the north and south pole and then midway between those two is the equator and then as i said we arbitrarily choose granite observatory for zero degrees longitude now if we project this grid up into the sky uh onto the celestial sphere we have celestial latitude and longitude so if i go back to one of these uh simulations this is one for terrestrial latitude and longitude and probably most of you are already familiar with this you can go a certain number of grease degrees to the south all the way down to 90 degrees south zero is the equator all the way up to 90 degrees north 45 degrees of course is midway and there's actually i remember correctly there's a town in oregon called half oregon because it's right at 45 degrees north so it's halfway between uh the equator and the north pole um so that is just latitude and longitude by the way the the the units here or the base you can either have a decimal with tenths of a degree and hundredths of a degree and so forth or often we use this base 60 sex adjustment system where you have minutes of arc you could have seconds of arc and just like with time keeping there are 60 arc minutes in a degree uh and 60 arc seconds in an arc minute that's just another way of recording um angles uh astronomers tend to use both you kind of run into them kind of equally although when you write a computer program to process angles you're going to want to use decimal angles not base 60 sex adjustable angles with arc minutes in arc seconds so kind of a pain you have to convert to decimal anyway as i was saying project these coordinates up into the sky and you have the celestial sphere this is a imaginary sphere that's concentric with the earth um project the earth's north pole up so that it intersects that sphere you've got the north celestial pole and then down at the bottom take the earth south pole projected up into space you've got the south celestial pole take the earth's equator project it up into space you've got the celestial equator now this particular module or a simulator allows us to look at the celestial sphere versus where the equator is horizontal versus the local horizon system where the horizon is horizontal right so here we're keeping zero altitude sort of horizontal as we kind of scroll around the celestials here um and then if i switch back to um the celestial system uh you know now we've got the equator horizontal and this is a nice way to show that the celestial system does not care about your latitude and longitude right so as i move around as i change my latitude that white circle on the earth the north celestial pole is fixed the south solicitor pole is fixed the celestial equator is fixed and this line here is supposed to represent the celestial prime meridian this is um the point of zero celestial longitude i'll come back to that in a second also being shown on here is the horizon circle which is that circle right there and that must be the east-west line so that would be a line going across the sky from east to west let's switch back here so you can see that yeah so that's a line that would go from east to through the zenith and then into the western horizon that doesn't have a name in classical astronomy this though allows you to see how the position of the north pole and the south pole change with your latitude right so as i walk around the curvature of the earth if i go to the north pole then the north celestial pole is directly over my head if i go to the equator then the celestial equator is directly over my head and if i go into the southern hemisphere i can't see the north celestial pole anymore but i can see the south celestial pole and if i go all the way to the south pole latitude negative 90 degrees there it would be directly over my head right there i would be at the south pole at the scott amundsen station in antarctica if i were at that point kind of at mid northern latitude if i kind of go to the typical latitude of new haven maybe like 40 degrees or so right about there there you can see where the celestial equator is coming out of the east horizon uh you can see where the north celestial pole is right there it turns out that the angle between the north celestial pole and the north horizon is equal to your latitude so if i were to make this closer to zero you can see there it's a small angle if i make this bigger i could go further around the curvature of the earth you can see it's quite steep and so the north solar pole is high above the north horizon and if i go into the southern hemisphere it goes below the horizon and so its altitude would be negative and i would have a southern latitude a negative latitude if you like i can actually switch back to there we go so see i'm my point my point on the earth is a little bit south of the equator here if i go down here there you see i'm quite far south of the equator if i switch back to the horizon mode there there's my horizon and then it flips over so the zenith is at the top so two different ways of showing the celestial sphere with the zenith your local zenith at the top that's going to depend on your latitude or with the north celestial pole at the top like so generally when you're looking at celestial latitude and longitude it makes more sense to orient the celestial sphere like this and if you're talking about local coordinates of altitude and azimuth makes more sense to orient it like this because then zero altitude is around the edge of the green ring around the edge of the green circle and the zenith is overhead the nader is beneath your feet and so forth now it's actually very convenient that there is a star that's very close to the north celestial pole so there's a star called polaris the north star uh not the brightest star in the sky but bright enough it's in the in the top 100 or so brightest stars it's bright enough you can see it pretty much anywhere so if you can find polaris that's going to tell you which way is due north so that's a useful uh navigation tip now if we're going to have a celestial analogy to latitude and longitude on the surface of the earth but projected up onto that celestial sphere we need reference planes so the celestial latitude it makes sense to use the celestial equator as the reference plane and of course that's what we do in astronomy so uh celestial latitude also known as declination so get used to this this is the term i'm going to use from now on declination it's zero on the celestial equator an object like a star or a planet or the sun is going to have declination zero when it's on the celestial equator and it's going to have declination 45 degrees if it's halfway to the north pole and it's going to have declination 90 degrees if it's right on the north celestial pole now this is not altitude this is not azimuth as the earth turns underneath um the stars do not care about the rotation of the earth do not care about your latitude and longitude stars essentially have a fixed declination on the sky as the earth is turning uh underneath you here now for celestial longitude again we need a reference point um how about the point directly above greenwich observatory that'll be zero degrees terrestrial longitude and zero zero degrees celestial longitude well the problem with that is that the earth turns so if the earth were turning uh 360 degrees in 24 hours that point above greenwich would spin around the celestial sphere and a star we want to have a star that has a fixed celestial latitude and longitude no matter how the earth is turning no matter what time it is on the earth or year a day in the year and so forth so we need a point that's fixed on the celestial sphere so we arbitrarily choose the point where the sun is on the spring equinox so over the course of a year the sun moves around the celestial sphere along the ecliptic and i have another video on that motion of the sun and the moon so the sun's moving along the ecliptic to the east once a year it crosses the celestial equator going from south to north we call it the spring equinox six months later it crosses the equator going from north to south so that's the fall equinox that when it has a declination of zero i could drag my star over there to let's say that star is representing the sun now that is by definition where the sun is on the spring equinox zero degrees celestial longitude now uh for historical reasons again we don't record celestial longitude in degrees we do it in hours of time so we take 360 degrees around the celestial equator and we divide it up into 24 hours because of course the of course the earth turns 360 degrees in 24 hours so a star at a certain right ascension a certain celestial longitude is going to be overhead at a certain time 24 hours 24 hours later the earth turns 360 degrees it'll be overhead once again so there is a way in which right ascension is connected to timekeeping so you just have to remember that one hour of time is equal to 15 degrees of angle so there you see we moved one hour around to the east let's move two hours around to the east also there we go two hours so uh 30 degrees uh so two hours would be 30 degrees around to the east go four hours around uh let's actually go six hours around now six is one quarter of 24 so that's actually 90 degrees around from the reference point for celestial longitude so uh that is celestial latitude and longitude has nothing to do with your latitude has nothing to do with time or anything on the surface of the earth it's what we think of as a fixed grid attached to the celestial sphere let me label some of these important reference circles so there's the celestial equator directly above the earth's equator north and south celestial poles so there's the north pole in the south pole in the earth there's the north celestial pole and the south celestial pole uh we'll just actually i'll just label the ones on the celestial sphere not on the earth the earth is turning to the east right ascension it turns out also increases to the east just like longitude on the surface of the earth by the way so if i increase my right ascension by dragging that slider to the right that star's right ascension increases so that's pretty straightforward if you were to um you know drag this sample star anywhere on the celestial sphere you know if you were to not show if you were to sort of hide the right ascension declination somehow you ought to be able to just sort of estimate its celestial latitude and longitude just as you should be able to estimate the celestial latitude and longitude of a city or a place on the earth just by looking at where it is relative to greenwich observatory in the prime meridian and the earth's equator which is zero latitude now what can get tricky what can be hard to understand is that when you think about where this star should be in your local coordinate system as the earth is turning and as you're looking at different latitudes and that's a little bit more complicated now this module right here which is the rotating sky explorer also on the university of nebraska website allows us to see both the celestial coordinate system with the north celestial pole at the top here and the local system with the zenith at the top at the same time so this is uh kind of convenient for trying to figure out how the system works so uh here i am at some random latitude let's actually plop down close to new haven although i don't care if i'm exactly at new haven so there you see where i am on the globe of the earth and if i were to put a star add star randomly okay there it is right there this would actually be an interesting test okay what's the celestial latitude and longitude of that star it's in the southern uh hemisphere of the celestial sphere so it's a negative declination i'd say maybe negative 10 negative 7 it is to the east of the celestial prime meridian that line looks like a yeah that's a 90 degree or six hour line so if we just look from above here that would be zero right ascension six hours right ascension 12 hours right ascension 18 hours right ascension and then back to zero right there so it looks like that star is like an hour shy of six hours so i'm gonna say declination uh negative seven and right ascension uh five hours all right okay if i click on it it gives me the data negative 6.6 degrees declination right ascension 5.2 hours right ascension relative to the to the celestial prime meridian okay where is that in my local sky all right well here's the celestial equator in the local sky there's the north celestial pole in the local sky uh i can't see the south celestial pole of course because the earth is in the way if i'm at latitude 36 degrees so there's the star down there now if i let time pass let's let the sky rotate as the earth orbit the earth rotates so as the earth rotates there it goes now at some point the star is above my horizon so there you see there it is right there so in terms of its uh azimuth and altitude there you see its azimuth is 180.9 it's pretty close to the local meridian and its altitude is 47.1 so let's let time pass right there's the earth turning its right ascension declination are not changing right those are essentially fixed but its altitude and azimuth are changing so now i can't see it below the horizon there are stars in the southern sky that you you might not be able to see at this latitude just because the earth is in the way so if i actually drag this guy down to there's negative negative 53.9 degrees declination now that guy does he actually can he get above the southern horizon he's making a circle around the south celestial pole right there here you see his altitude is negative 60. here it is coming up getting close to the horizon uh let's see if it clears the horizon just barely doesn't clear the horizon it got to negative 0.2 or something like that degrees altitude so that's a star that's a part of the celestial sphere i would never be able to see from that altitude there are stars down there like the southern cross um all the constellations of the southern hemisphere that i can't see from that latitude so this is kind of a problem in observational astronomy is you want to know an object with a particular right ascension and declination how high is it going to get above my horizon well it turns out there's a simple relationship between a star's declination and your latitude and how high it will get above the horizon so for example if i put this star at zero degrees declination right there it's going to be on the celestial equator and it turns out that the celestial equator crosses the local meridian that point right there it crosses at a point which is equal to an altitude of 90 degrees minus your latitude so i said that the altitude of the north pole is plus 90 degrees there it is right there or plus 90 degrees it's equal to your latitude what i meant to say so this is uh this is your latitude right here from the north pole to the equator has to be 90 degrees that's always the case pole to equator has to be 90 degrees so then from north horizon to south horizon has to be 180 degrees so we've got your latitude we've got 90 degrees we've got this angle from the celestial equator to the horizon that has to be 90 degrees minus your latitude so uh if we let time pass again and get that star close to see spin it around actually we'll just drag it to there so cheat a little bit so there it is close to the local meridian and its altitude is 53.1 degrees so uh that has to be equal to 90 degrees minus my latitude yeah that's very close not exactly but that's very close i'm not exactly on it but that's very close to 90 degrees minus my latitude now if the star had a negative declination let me drag it down here by about 10 degrees or so all right put it right about there it's going to be 10 degrees south of the equator in my local sky if i take it 10 degrees north of the celestial equator now it's about 10 degrees drag it around a little bit now it's about 10 degrees north of my equator in my local sky so it's going to appear 10 degrees higher in the sky when it transits if it's 10 degrees north it's going to be 10 degrees lower in the sky at an altitude 10 degrees less if its declination is negative 10 degrees so that's the connection between declination and then altitude in your local sky and your local latitude all right that's pretty complicated so i recommend that you kind of play around with this in your in your spare time drop a few stars on the celestial sphere and make sure you understand the relationship about how their celestial latitude and longitude their right ascension declination uh affects how they appear to move in your local sky right so we can actually run this here and so you can see that star right there which is at fairly high declination 57.5 degrees is in my northern sky and there you can see its altitude and azimuth changing as it goes around the north celestial pole as the earth is rotating that area that you can never see at your latitude i can highlight that right here there it is right there at that latitude i would never i'll never be able to see the stars on that part of the celestial sphere conversely there's a part of the celestial sphere in the north where the stars never set so that's called circumpolar those stars are circumpolar up there and i can figure out where that is based on my latitude right it's going to be a declination of 90 minus my latitude so that those declinations are always going to be circumpolar now let's see what this looks like in the planetarium simulator so here i am actually in new haven and i want to see that right ascension and declination the celestial latitude and longitude coordinates so i'm going to go to the settings again sky and viewing options i'm going to turn on the equator uh let's see i'm going to turn on i'll turn on the equatorial grid for now i'm going to turn it off again in a second so there is the celestial sphere with right ascension coordinates and declination coordinates indicated so the equator has declination zero here's uh let's see what's that plus 10 yeah there it is mark there plus 10 plus 20 plus 30 and so forth all the way up minus 10 minus uh 20 minus 30 and so forth all the way down and then there's a region that i wouldn't be able to see from new haven now this is a simulation of the sky so if i want to turn off the ground i have that option all i have to do is go down here and hit the g key or click on that icon and the ground disappears now i can see the south celestial pole right there and all of these stars down here that i would not be able to see i'm going to turn the ground back on for now um now i selected uh vega where is vega in the sky right now there it is over there now vega also has a particular right ascension declination so let's turn that back on in the information box here so we'll say right ascension declination like so so now you can see there's its right ascension declination for the year 2000 18 hours 36 minutes and some seconds 38 degrees uh 47 arc minutes and some seconds so that tells you where it is relative to the equator and relative to that arbitrary point where the sun is on the spring equinox so if i let time pass again let's speed time up this is a very important point the altitude and azimuth of the star are changing the altitude's going down the azimuth is increasing but the right ascension and declination are not changing because the grid is rotating and carrying the stars with them now that's not literally a hundred percent true um the stars do move around relative to each other just because we're all orbiting around the center of the milky way i'm going to turn off the atmosphere so we can see the stars here even when the sun's up the stars are of course moving through space as we all orbit around the center of the milky way so their positions relative to each other will change that's a motion called proper motion it's very very slow so most of the time you don't have to worry about it unless you're worried about the positions of the stars over long periods of time another effect is that the coordinate grid does move due to an effect called precession precession of the equinoxes and i have another video about that as well the precession effect means that you need to know the date for the coordinates so right ascension declination for 2000 those are going to be fixed but if i wanted to know right ascension declination for this year or this moment this will change very very slowly because the the coordinate grid does actually move around over time okay so i can look to the north and i can see the north celestial pole right there and i can see the star polaris right there it's not exactly on the north celestial pole but it's close enough to be useful for navigation um i can see the celestial equator coming out of the east horizon right there it crosses the local meridian right there at an altitude altitude equal to 90 degrees minus my latitude uh and then it goes down here into the western horizon over there all right now uh any asteroid planet star whatever galaxy quasar is going to have a particular right ascension declination and those are the coordinates you're going to use to point your telescope so you calibrate your telescope with some known star or known object and then you can move the telescope a certain angle north south east west and find that object okay one last thing i want to say about these two coordinate systems and especially using stellarium stellarium is kind of designed for amateur astronomers who are planning observations and by default it's in this local coordinate system coordinate mode where the horizon stays horizontal right and i look straight overhead to z to see the zenith a mode that's useful when you're looking at the celestial sphere is the so-called equatorial motor i call it equatorial mount mode so i would hit command m to switch that with the keyboard but i can also just click on the little telescope icon right there and it changes the orientation of the sky so then instead of having a horizon that's horizontal you see it's tilted now stellarium is keeping the celestial equator horizontal like so so now when i look overhead i see the north celestial pole if i look beneath my feet well directly beneath my feet would be the south celestial pole i have the ground in the way so this might be a good time to turn off the ground there we go so this is uh now as if maybe the earth didn't exist or the earth were transparent i look beneath my feet and there's the south celestial pole i look horizontally and there's the celestial equator east is to the left west is to the right north is up south is down and then i look overhead and there's the north celestial pole up there all right oh i just mentioned that thing about east to the left west to the right and so forth that's kind of a useful thing to know when you're looking at star maps for example here if we look at the constellation of orion and i'm just going to zoom in on it a little bit there you see the meridian going by i'll just hide that for now so there's the constellation of orion if i look at the star rigel down here the lower right foot you can see okay it has a um it's hard to see the the coordinates let me turn off the grid for a second so we can see those coordinates a little bit better oh i have to turn it off i have to turn it off over here to turn it off right there okay so you can see those coordinates so it's at 5 hours and 14 minutes and then negative 8 degrees declination 12 arc minutes so it's south of the celestial equator um versus the star over here of battle jews so battle jews if north is up uh betelgeuse is kind of to the left of uh rigel on the celestial sphere and it's up uh it's north it's sort of closer to the north celestial pole you can kind of figure out these north-south east-west directions by looking at the coordinates so rigel here has right ascension i just said five hours of 14 minutes declination negative 8 degrees 12 minutes battle jews is north of the celestial equator so it's got a declination of plus 7 degrees 24 arc minutes and a right ascension of 5 hours 55 minutes its right ascension is bigger so it is to the east on the celestial sphere of rigel its declination is more positive so it is north of rigel on the celestial sphere so that's kind of how astronomers use directions notice that north is up south is down but east is to the left and west is to the right that's kind of the opposite of how you normally look at a map right and of course the reason why is because when you look at a map you're looking at the outside of the sphere of the earth when you look at the celestial sphere you're looking at the inside of the sphere so even though east and west are in the same directions uh on the spheres themselves if you look at the inside of the sphere east and west end up being reversed it's something that astronomers are often confused about all right so that's a lot of information about celestial coordinates uh i'm going to review this in a couple of different ways but these are the the most important basic things so uh right ascension declination celestial longitude and latitude it's catalogued for all the objects in the sky and then your local system then is altitude and azimuth just the height above the horizon and the angle relative to north and we use both of these angles we both use both of these coordinate systems to measure positions of objects in the sky in observational astronomy

this is a tutorial video on celestial coordinates specifically the local coordinates of altitude and azimuth as well as the celestial coordinates or the equatorial coordinates of right ascension and declination what we should really call celestial latitude and longitude but astronomers call them right ascension and declination i&#39;m going to be uh going over these coordinate systems in the program stellarium so this is also a bit of an introduction on some of the features of the planetary uh planetarium program stellarium all right so uh first of all let&#39;s talk about those local coordinates altitude and azimuth um altitude is just the angle that something makes in the sky relative to the horizon uh altitude sounds like something uh you would measure on a plane altimeter and it would have units of meters it would be the height above the the ground or the sea level uh that&#39;s not what astronomers mean when they say altitude it&#39;s an angle so it&#39;s going to be measured in degrees i think we could measure it in radians or some other unit but degrees degrees arc minutes arc seconds and it&#39;s the angle that the object makes with the horizon so i there you see the sun kind of over in the east and if i actually click on it stellarium will give me information about its altitude and azimuth by default it actually gives you a lot more information but one of the things i&#39;ve done here if you go into the configuration window for stellarium you can you can control how much information you see when you click on an object so if i click on that and or you hit f2 key and if i go to this information tab right here then selected object information i just want to see the name and its altitude and azimuth for right now so you see the altitude of the sun is 47 degrees 59 arc minutes and some mark seconds and when this turns over to zero the next degree marker goes up so it&#39;s at 48 degrees above the horizon right there now the horizon has an altitude of zero so when an object is rising or setting its altitude is zero and then the highest altitude you can have is directly overhead that&#39;s what astronomers call the zenith or zenith and that&#39;s an altitude of plus 90 degrees you could have negative altitude which would mean the object is below the horizon so you normally wouldn&#39;t be able to see it and that&#39;s perfectly fine and then at negative 90 degrees would be what we call the nadir nadir opposite the zenith so uh the reference plane for this local coordinate system of altitude and azimuth is the horizon um the other uh angle is the azimuth and azimuth is just where along the horizon is the point directly below or above below if it&#39;s a if the object has a positive altitude so that&#39;s it&#39;s a above the horizon and and then the point that would be above an object that was negative altitude below the horizon uh so you drop down to the point on the horizon uh below that up below where that object is like there&#39;s the sun right there i just drop straight down and then that intersects the horizon somewhere and now i can measure its azimuth azimuth is really just a compass heading compass bearing around the horizon from the reference point which is due north so due north is 0 degrees azimuth due east is 90 degrees azimuth due south is 180 degrees as azimuth and then due west is 270 degrees azimuth uh and then when you get around to north again it resets to 360. so you get the 358 359 0 degrees azimuth so if we look back here at the sun there&#39;s the sun right there i dropped straight down it&#39;s now 48 degrees above the horizon right there and it&#39;s azimuth is should be a little bit more than 90 degrees because it&#39;s a little bit south of due east and you see its azimuth is 101 degrees right there i actually have a plug-in in stellarium that lets me show the compass marks so if i turn that on you can see oh yeah there are the azimuth angles from zero to the north 90 to the east 180 to the south and so forth so that&#39;s altitude and azimuth um it&#39;s kind of like the coordinates you might put into a cannon if you were going to aim it somewhere you&#39;d say okay elevation or altitude 45 degrees above the horizon you turn a big crank and it goes the gun goes up to a steeper steeper angle and you say heading 165 degrees and then you turn another crank and it turns and points in that direction and so those are the local coordinates there&#39;s an important reference line for the local system which is called the local meridian that is a line which goes from due south directly overhead through the zenith and then down to due north and intersects the horizon due north so let me turn that on if i go over here to sky and viewing options window or hit the f4 key one of the options is markings and you see here celestial sphere i can draw a meridian line so now there&#39;s a line right there kind of greenish that goes from 180 through the zenith up there and then if i turn around to the north it goes into the north horizon now technically there&#39;s an upper meridian and a lower meridian um but for most purposes it doesn&#39;t matter you can just think of the meridian as going from south to overhead to north and that&#39;s that&#39;s what&#39;s important now the meridian is a useful reference line because everything uh that rises and sets through the sky has to cross the meridian at some point um if i speed time up you can see that there&#39;s the sun going across the sky from east to west some point the sun sets over there and uh the earth is spinning way too fast there goes the sun again across the sky um you see i still am showing its altitude and azimuth there it&#39;s negative it&#39;s below the horizon there when it&#39;s positive it&#39;s above the horizon you see the altitude is increasing until it gets to 360 and then it flips around to zero that&#39;s true for all the stars as well let me slow this down a little bit here all right so there&#39;s the sun crossing the meridian that&#39;s going to be local solar noon that&#39;s when it&#39;s going to be at its highest possible altitude and of course its azimuth has to be exactly 180 degrees because the meridian everything on the meridian has an azimuth of 180 degrees if it&#39;s south of the zenith or zero degrees if it&#39;s north of the zenith let&#39;s see when the sun goes down again we&#39;ll look at a couple of different stars so let&#39;s watch the sun go down the sun&#39;s setting north of due west because it&#39;s summertime and there are the planets that are out tonight and then here are some bright stars let me just bring the playback back to normal speed here and there&#39;s the star vega right there now vega you can see has an azimuth of 75 degrees so it&#39;s kind of a basically east a little bit north of east at that point in time right there&#39;s nine degrees is due east let me go back to the uh the direction markers like so so there&#39;s due east right there and it&#39;s altitude&#39;s pretty high it&#39;s at 50 degrees and 29 arc minutes and as time passes its altitude gets higher and higher and if i speed time up again speed time off by 10 100 a thousand we can watch its altitude get higher and higher until it crosses the meridian so here at some point we&#39;ll pause when i get close all right right about there i didn&#39;t get it exactly but i&#39;m close it&#39;s an azimuth 176 and 54 arc minutes increasing and there you see its altitude 87 degrees and 29 arc minutes oh it&#39;s very close to 90 degrees at this particular latitude in new haven vega almost passes through the zenith but not quite exactly right not quite exactly so it&#39;s still going up very gradually because i&#39;m not quite at the meridian once it crosses the meridian this number is going to stop going up and start going down so uh astronomers generally want to observe an object when it&#39;s as close to the meridian as possible because we want to observe objects when they&#39;re at their highest possible altitude that is because when they&#39;re at the high altitude uh you&#39;re looking through less atmosphere if we look at an object that&#39;s down here low at low altitude close to the horizon we&#39;re looking through maybe two or three times as much atmosphere as when we look overhead and the atmosphere has the effect of dimming objects that&#39;s why the sun is so much dimmer when it&#39;s rising or setting it also will redden objects so just like the sun looks very red at sunrise and sunset uh because the atmosphere is preferentially scattering blue light so the sun is uh the light from the sun is being scattered to make the sky blue somewhere west of you when you see a nice red sunset the other effect that the atmosphere has is that the turbulence in the atmosphere will cause the stars to twinkle or shimmer if you like and that will blur out your view of the universe when you&#39;re looking through extra atmosphere i mean you&#39;d love to have uh unlimited time on the hubble space telescope so you can not look through any atmosphere but here with a ground based telescope you have to minimize the atmosphere you look through so you want to look when it&#39;s as high in the sky as possible the highest possible altitude now you can see vegas crossed the meridian so the number the altitude here is slowly decreasing so um let me just to review that turn on the altitude and azimuth grid so if i go down here however to hit the z key stellarium turns on this altitude and azimuth grid so you can see these wedges that are coming up and converging at the zenith those are wedges of particular azimuth constant azimuth and then these circles that go up from the horizon to the zenith those are circles of constant altitude so that&#39;s 10 degrees right there 20 degrees 30 degrees and so forth all the way up to 90 degrees and then you can see the same with the um the azimuth markings here as the earth turns the stars will change their altitude and azimuth as they rise in the east cross the meridian reach a maximum altitude and then start setting in the west i have a nice uh kind of simulation of this as seen from outside the celestial sphere uh outside the dome of the sky now this is of course an impossible thing to do um but this is kind of a convenient model to have this is based on the university of nebraska astronomy simulations you can go to this website to see the the animations and simulations that they made for their astronomy classes at university of nebraska and they put these online they&#39;re very useful for teaching intra astronomy anyway this one is the azimuth altitude demonstrator and you see a little stick figure there in the middle of his horizon and you can see north south east west marked on the horizon you can always figure these out because by definition the direction the earth turns is east so this must be the direction the earth has to turn and then 90 degrees to the left of that is north 90 degrees to the right of that is south and so forth so you can see those reference points you can see the meridian right here going from south to overhead to north there&#39;s the zenith there&#39;s the nadir and you can just see uh the altitude 45 degrees uh up to close to 90 degrees down to 10 degrees or less that can be negative if it&#39;s below the horizon uh and then you can also see the changing azimuth right here right so if i drag the star around to the north the azimuth decreases like so and if i drag it back around to the south the azimuth increases if it were exactly on the meridian its azimuth would be exactly 180 degrees and so i can specify any point in the sky by using these two angles with zero altitude at the horizon and zero azimuth at the north horizon due north on the north horizon if we go to zero azimuth and zero altitude that should be exactly on the north horizon dial that in let&#39;s put zero in here in fact there we go so there&#39;s that a star that we&#39;re at that point would be zero altitude and zero azimuth all right um well let&#39;s move on to talk about celestial latitude and longitude uh this is a coordinate system which doesn&#39;t actually depend on your local horizon this is a system that&#39;s based on the celestial equator and the celestial north and south pole as well as the ecliptic the path the sun takes uh in the sky so uh celestial latitude and longitude or um in astronomy we call them declination for celestial latitude and right ascension for celestial longitude uh they&#39;re defined very much in analogy with terrestrial latitude and longitude uh so every point on the surface of the earth has a particular latitude and longitude and the latitude is measured from the equator around the curvature of the earth so the equator would be zero latitude the north pole would be plus 90 degrees north or just 90 degrees north the south pole would be negative 90 degrees or just 90 degrees south and then there&#39;s also a longitude which is the east-west coordinate now there&#39;s no good place to define zero degrees longitude it&#39;s completely uh arbitrary um but uh we have by convention chosen london or actually the greenwich observatory uh as the reference point as for zero degrees uh longitude so uh i can actually turn on the latitude and longitude system uh here in google earth if i go up here to view grid right there you can see the latitude and longitude for everything on the earth so zero degrees is the prime meridian which goes through greenwich observatory ah an observatory uh there must be an astronomical reason for choosing that point for zero degrees longitude uh there is uh maybe i&#39;ll talk about that in a separate video about time keeping and celestial navigation uh but that&#39;s just what we&#39;ve chosen by convention and then zero degrees latitude is the earth&#39;s equator which is of course midway between the north and the south pole so in a sense the latitude and longitude grid is defined by the rotation of the earth because the earth rotates on its axis that defines the north the north and south pole and then midway between those two is the equator and then as i said we arbitrarily choose granite observatory for zero degrees longitude now if we project this grid up into the sky uh onto the celestial sphere we have celestial latitude and longitude so if i go back to one of these uh simulations this is one for terrestrial latitude and longitude and probably most of you are already familiar with this you can go a certain number of grease degrees to the south all the way down to 90 degrees south zero is the equator all the way up to 90 degrees north 45 degrees of course is midway and there&#39;s actually i remember correctly there&#39;s a town in oregon called half oregon because it&#39;s right at 45 degrees north so it&#39;s halfway between uh the equator and the north pole um so that is just latitude and longitude by the way the the the units here or the base you can either have a decimal with tenths of a degree and hundredths of a degree and so forth or often we use this base 60 sex adjustment system where you have minutes of arc you could have seconds of arc and just like with time keeping there are 60 arc minutes in a degree uh and 60 arc seconds in an arc minute that&#39;s just another way of recording um angles uh astronomers tend to use both you kind of run into them kind of equally although when you write a computer program to process angles you&#39;re going to want to use decimal angles not base 60 sex adjustable angles with arc minutes in arc seconds so kind of a pain you have to convert to decimal anyway as i was saying project these coordinates up into the sky and you have the celestial sphere this is a imaginary sphere that&#39;s concentric with the earth um project the earth&#39;s north pole up so that it intersects that sphere you&#39;ve got the north celestial pole and then down at the bottom take the earth south pole projected up into space you&#39;ve got the south celestial pole take the earth&#39;s equator project it up into space you&#39;ve got the celestial equator now this particular module or a simulator allows us to look at the celestial sphere versus where the equator is horizontal versus the local horizon system where the horizon is horizontal right so here we&#39;re keeping zero altitude sort of horizontal as we kind of scroll around the celestials here um and then if i switch back to um the celestial system uh you know now we&#39;ve got the equator horizontal and this is a nice way to show that the celestial system does not care about your latitude and longitude right so as i move around as i change my latitude that white circle on the earth the north celestial pole is fixed the south solicitor pole is fixed the celestial equator is fixed and this line here is supposed to represent the celestial prime meridian this is um the point of zero celestial longitude i&#39;ll come back to that in a second also being shown on here is the horizon circle which is that circle right there and that must be the east-west line so that would be a line going across the sky from east to west let&#39;s switch back here so you can see that yeah so that&#39;s a line that would go from east to through the zenith and then into the western horizon that doesn&#39;t have a name in classical astronomy this though allows you to see how the position of the north pole and the south pole change with your latitude right so as i walk around the curvature of the earth if i go to the north pole then the north celestial pole is directly over my head if i go to the equator then the celestial equator is directly over my head and if i go into the southern hemisphere i can&#39;t see the north celestial pole anymore but i can see the south celestial pole and if i go all the way to the south pole latitude negative 90 degrees there it would be directly over my head right there i would be at the south pole at the scott amundsen station in antarctica if i were at that point kind of at mid northern latitude if i kind of go to the typical latitude of new haven maybe like 40 degrees or so right about there there you can see where the celestial equator is coming out of the east horizon uh you can see where the north celestial pole is right there it turns out that the angle between the north celestial pole and the north horizon is equal to your latitude so if i were to make this closer to zero you can see there it&#39;s a small angle if i make this bigger i could go further around the curvature of the earth you can see it&#39;s quite steep and so the north solar pole is high above the north horizon and if i go into the southern hemisphere it goes below the horizon and so its altitude would be negative and i would have a southern latitude a negative latitude if you like i can actually switch back to there we go so see i&#39;m my point my point on the earth is a little bit south of the equator here if i go down here there you see i&#39;m quite far south of the equator if i switch back to the horizon mode there there&#39;s my horizon and then it flips over so the zenith is at the top so two different ways of showing the celestial sphere with the zenith your local zenith at the top that&#39;s going to depend on your latitude or with the north celestial pole at the top like so generally when you&#39;re looking at celestial latitude and longitude it makes more sense to orient the celestial sphere like this and if you&#39;re talking about local coordinates of altitude and azimuth makes more sense to orient it like this because then zero altitude is around the edge of the green ring around the edge of the green circle and the zenith is overhead the nader is beneath your feet and so forth now it&#39;s actually very convenient that there is a star that&#39;s very close to the north celestial pole so there&#39;s a star called polaris the north star uh not the brightest star in the sky but bright enough it&#39;s in the in the top 100 or so brightest stars it&#39;s bright enough you can see it pretty much anywhere so if you can find polaris that&#39;s going to tell you which way is due north so that&#39;s a useful uh navigation tip now if we&#39;re going to have a celestial analogy to latitude and longitude on the surface of the earth but projected up onto that celestial sphere we need reference planes so the celestial latitude it makes sense to use the celestial equator as the reference plane and of course that&#39;s what we do in astronomy so uh celestial latitude also known as declination so get used to this this is the term i&#39;m going to use from now on declination it&#39;s zero on the celestial equator an object like a star or a planet or the sun is going to have declination zero when it&#39;s on the celestial equator and it&#39;s going to have declination 45 degrees if it&#39;s halfway to the north pole and it&#39;s going to have declination 90 degrees if it&#39;s right on the north celestial pole now this is not altitude this is not azimuth as the earth turns underneath um the stars do not care about the rotation of the earth do not care about your latitude and longitude stars essentially have a fixed declination on the sky as the earth is turning uh underneath you here now for celestial longitude again we need a reference point um how about the point directly above greenwich observatory that&#39;ll be zero degrees terrestrial longitude and zero zero degrees celestial longitude well the problem with that is that the earth turns so if the earth were turning uh 360 degrees in 24 hours that point above greenwich would spin around the celestial sphere and a star we want to have a star that has a fixed celestial latitude and longitude no matter how the earth is turning no matter what time it is on the earth or year a day in the year and so forth so we need a point that&#39;s fixed on the celestial sphere so we arbitrarily choose the point where the sun is on the spring equinox so over the course of a year the sun moves around the celestial sphere along the ecliptic and i have another video on that motion of the sun and the moon so the sun&#39;s moving along the ecliptic to the east once a year it crosses the celestial equator going from south to north we call it the spring equinox six months later it crosses the equator going from north to south so that&#39;s the fall equinox that when it has a declination of zero i could drag my star over there to let&#39;s say that star is representing the sun now that is by definition where the sun is on the spring equinox zero degrees celestial longitude now uh for historical reasons again we don&#39;t record celestial longitude in degrees we do it in hours of time so we take 360 degrees around the celestial equator and we divide it up into 24 hours because of course the of course the earth turns 360 degrees in 24 hours so a star at a certain right ascension a certain celestial longitude is going to be overhead at a certain time 24 hours 24 hours later the earth turns 360 degrees it&#39;ll be overhead once again so there is a way in which right ascension is connected to timekeeping so you just have to remember that one hour of time is equal to 15 degrees of angle so there you see we moved one hour around to the east let&#39;s move two hours around to the east also there we go two hours so uh 30 degrees uh so two hours would be 30 degrees around to the east go four hours around uh let&#39;s actually go six hours around now six is one quarter of 24 so that&#39;s actually 90 degrees around from the reference point for celestial longitude so uh that is celestial latitude and longitude has nothing to do with your latitude has nothing to do with time or anything on the surface of the earth it&#39;s what we think of as a fixed grid attached to the celestial sphere let me label some of these important reference circles so there&#39;s the celestial equator directly above the earth&#39;s equator north and south celestial poles so there&#39;s the north pole in the south pole in the earth there&#39;s the north celestial pole and the south celestial pole uh we&#39;ll just actually i&#39;ll just label the ones on the celestial sphere not on the earth the earth is turning to the east right ascension it turns out also increases to the east just like longitude on the surface of the earth by the way so if i increase my right ascension by dragging that slider to the right that star&#39;s right ascension increases so that&#39;s pretty straightforward if you were to um you know drag this sample star anywhere on the celestial sphere you know if you were to not show if you were to sort of hide the right ascension declination somehow you ought to be able to just sort of estimate its celestial latitude and longitude just as you should be able to estimate the celestial latitude and longitude of a city or a place on the earth just by looking at where it is relative to greenwich observatory in the prime meridian and the earth&#39;s equator which is zero latitude now what can get tricky what can be hard to understand is that when you think about where this star should be in your local coordinate system as the earth is turning and as you&#39;re looking at different latitudes and that&#39;s a little bit more complicated now this module right here which is the rotating sky explorer also on the university of nebraska website allows us to see both the celestial coordinate system with the north celestial pole at the top here and the local system with the zenith at the top at the same time so this is uh kind of convenient for trying to figure out how the system works so uh here i am at some random latitude let&#39;s actually plop down close to new haven although i don&#39;t care if i&#39;m exactly at new haven so there you see where i am on the globe of the earth and if i were to put a star add star randomly okay there it is right there this would actually be an interesting test okay what&#39;s the celestial latitude and longitude of that star it&#39;s in the southern uh hemisphere of the celestial sphere so it&#39;s a negative declination i&#39;d say maybe negative 10 negative 7 it is to the east of the celestial prime meridian that line looks like a yeah that&#39;s a 90 degree or six hour line so if we just look from above here that would be zero right ascension six hours right ascension 12 hours right ascension 18 hours right ascension and then back to zero right there so it looks like that star is like an hour shy of six hours so i&#39;m gonna say declination uh negative seven and right ascension uh five hours all right okay if i click on it it gives me the data negative 6.6 degrees declination right ascension 5.2 hours right ascension relative to the to the celestial prime meridian okay where is that in my local sky all right well here&#39;s the celestial equator in the local sky there&#39;s the north celestial pole in the local sky uh i can&#39;t see the south celestial pole of course because the earth is in the way if i&#39;m at latitude 36 degrees so there&#39;s the star down there now if i let time pass let&#39;s let the sky rotate as the earth orbit the earth rotates so as the earth rotates there it goes now at some point the star is above my horizon so there you see there it is right there so in terms of its uh azimuth and altitude there you see its azimuth is 180.9 it&#39;s pretty close to the local meridian and its altitude is 47.1 so let&#39;s let time pass right there&#39;s the earth turning its right ascension declination are not changing right those are essentially fixed but its altitude and azimuth are changing so now i can&#39;t see it below the horizon there are stars in the southern sky that you you might not be able to see at this latitude just because the earth is in the way so if i actually drag this guy down to there&#39;s negative negative 53.9 degrees declination now that guy does he actually can he get above the southern horizon he&#39;s making a circle around the south celestial pole right there here you see his altitude is negative 60. here it is coming up getting close to the horizon uh let&#39;s see if it clears the horizon just barely doesn&#39;t clear the horizon it got to negative 0.2 or something like that degrees altitude so that&#39;s a star that&#39;s a part of the celestial sphere i would never be able to see from that altitude there are stars down there like the southern cross um all the constellations of the southern hemisphere that i can&#39;t see from that latitude so this is kind of a problem in observational astronomy is you want to know an object with a particular right ascension and declination how high is it going to get above my horizon well it turns out there&#39;s a simple relationship between a star&#39;s declination and your latitude and how high it will get above the horizon so for example if i put this star at zero degrees declination right there it&#39;s going to be on the celestial equator and it turns out that the celestial equator crosses the local meridian that point right there it crosses at a point which is equal to an altitude of 90 degrees minus your latitude so i said that the altitude of the north pole is plus 90 degrees there it is right there or plus 90 degrees it&#39;s equal to your latitude what i meant to say so this is uh this is your latitude right here from the north pole to the equator has to be 90 degrees that&#39;s always the case pole to equator has to be 90 degrees so then from north horizon to south horizon has to be 180 degrees so we&#39;ve got your latitude we&#39;ve got 90 degrees we&#39;ve got this angle from the celestial equator to the horizon that has to be 90 degrees minus your latitude so uh if we let time pass again and get that star close to see spin it around actually we&#39;ll just drag it to there so cheat a little bit so there it is close to the local meridian and its altitude is 53.1 degrees so uh that has to be equal to 90 degrees minus my latitude yeah that&#39;s very close not exactly but that&#39;s very close i&#39;m not exactly on it but that&#39;s very close to 90 degrees minus my latitude now if the star had a negative declination let me drag it down here by about 10 degrees or so all right put it right about there it&#39;s going to be 10 degrees south of the equator in my local sky if i take it 10 degrees north of the celestial equator now it&#39;s about 10 degrees drag it around a little bit now it&#39;s about 10 degrees north of my equator in my local sky so it&#39;s going to appear 10 degrees higher in the sky when it transits if it&#39;s 10 degrees north it&#39;s going to be 10 degrees lower in the sky at an altitude 10 degrees less if its declination is negative 10 degrees so that&#39;s the connection between declination and then altitude in your local sky and your local latitude all right that&#39;s pretty complicated so i recommend that you kind of play around with this in your in your spare time drop a few stars on the celestial sphere and make sure you understand the relationship about how their celestial latitude and longitude their right ascension declination uh affects how they appear to move in your local sky right so we can actually run this here and so you can see that star right there which is at fairly high declination 57.5 degrees is in my northern sky and there you can see its altitude and azimuth changing as it goes around the north celestial pole as the earth is rotating that area that you can never see at your latitude i can highlight that right here there it is right there at that latitude i would never i&#39;ll never be able to see the stars on that part of the celestial sphere conversely there&#39;s a part of the celestial sphere in the north where the stars never set so that&#39;s called circumpolar those stars are circumpolar up there and i can figure out where that is based on my latitude right it&#39;s going to be a declination of 90 minus my latitude so that those declinations are always going to be circumpolar now let&#39;s see what this looks like in the planetarium simulator so here i am actually in new haven and i want to see that right ascension and declination the celestial latitude and longitude coordinates so i&#39;m going to go to the settings again sky and viewing options i&#39;m going to turn on the equator uh let&#39;s see i&#39;m going to turn on i&#39;ll turn on the equatorial grid for now i&#39;m going to turn it off again in a second so there is the celestial sphere with right ascension coordinates and declination coordinates indicated so the equator has declination zero here&#39;s uh let&#39;s see what&#39;s that plus 10 yeah there it is mark there plus 10 plus 20 plus 30 and so forth all the way up minus 10 minus uh 20 minus 30 and so forth all the way down and then there&#39;s a region that i wouldn&#39;t be able to see from new haven now this is a simulation of the sky so if i want to turn off the ground i have that option all i have to do is go down here and hit the g key or click on that icon and the ground disappears now i can see the south celestial pole right there and all of these stars down here that i would not be able to see i&#39;m going to turn the ground back on for now um now i selected uh vega where is vega in the sky right now there it is over there now vega also has a particular right ascension declination so let&#39;s turn that back on in the information box here so we&#39;ll say right ascension declination like so so now you can see there&#39;s its right ascension declination for the year 2000 18 hours 36 minutes and some seconds 38 degrees uh 47 arc minutes and some seconds so that tells you where it is relative to the equator and relative to that arbitrary point where the sun is on the spring equinox so if i let time pass again let&#39;s speed time up this is a very important point the altitude and azimuth of the star are changing the altitude&#39;s going down the azimuth is increasing but the right ascension and declination are not changing because the grid is rotating and carrying the stars with them now that&#39;s not literally a hundred percent true um the stars do move around relative to each other just because we&#39;re all orbiting around the center of the milky way i&#39;m going to turn off the atmosphere so we can see the stars here even when the sun&#39;s up the stars are of course moving through space as we all orbit around the center of the milky way so their positions relative to each other will change that&#39;s a motion called proper motion it&#39;s very very slow so most of the time you don&#39;t have to worry about it unless you&#39;re worried about the positions of the stars over long periods of time another effect is that the coordinate grid does move due to an effect called precession precession of the equinoxes and i have another video about that as well the precession effect means that you need to know the date for the coordinates so right ascension declination for 2000 those are going to be fixed but if i wanted to know right ascension declination for this year or this moment this will change very very slowly because the the coordinate grid does actually move around over time okay so i can look to the north and i can see the north celestial pole right there and i can see the star polaris right there it&#39;s not exactly on the north celestial pole but it&#39;s close enough to be useful for navigation um i can see the celestial equator coming out of the east horizon right there it crosses the local meridian right there at an altitude altitude equal to 90 degrees minus my latitude uh and then it goes down here into the western horizon over there all right now uh any asteroid planet star whatever galaxy quasar is going to have a particular right ascension declination and those are the coordinates you&#39;re going to use to point your telescope so you calibrate your telescope with some known star or known object and then you can move the telescope a certain angle north south east west and find that object okay one last thing i want to say about these two coordinate systems and especially using stellarium stellarium is kind of designed for amateur astronomers who are planning observations and by default it&#39;s in this local coordinate system coordinate mode where the horizon stays horizontal right and i look straight overhead to z to see the zenith a mode that&#39;s useful when you&#39;re looking at the celestial sphere is the so-called equatorial motor i call it equatorial mount mode so i would hit command m to switch that with the keyboard but i can also just click on the little telescope icon right there and it changes the orientation of the sky so then instead of having a horizon that&#39;s horizontal you see it&#39;s tilted now stellarium is keeping the celestial equator horizontal like so so now when i look overhead i see the north celestial pole if i look beneath my feet well directly beneath my feet would be the south celestial pole i have the ground in the way so this might be a good time to turn off the ground there we go so this is uh now as if maybe the earth didn&#39;t exist or the earth were transparent i look beneath my feet and there&#39;s the south celestial pole i look horizontally and there&#39;s the celestial equator east is to the left west is to the right north is up south is down and then i look overhead and there&#39;s the north celestial pole up there all right oh i just mentioned that thing about east to the left west to the right and so forth that&#39;s kind of a useful thing to know when you&#39;re looking at star maps for example here if we look at the constellation of orion and i&#39;m just going to zoom in on it a little bit there you see the meridian going by i&#39;ll just hide that for now so there&#39;s the constellation of orion if i look at the star rigel down here the lower right foot you can see okay it has a um it&#39;s hard to see the the coordinates let me turn off the grid for a second so we can see those coordinates a little bit better oh i have to turn it off i have to turn it off over here to turn it off right there okay so you can see those coordinates so it&#39;s at 5 hours and 14 minutes and then negative 8 degrees declination 12 arc minutes so it&#39;s south of the celestial equator um versus the star over here of battle jews so battle jews if north is up uh betelgeuse is kind of to the left of uh rigel on the celestial sphere and it&#39;s up uh it&#39;s north it&#39;s sort of closer to the north celestial pole you can kind of figure out these north-south east-west directions by looking at the coordinates so rigel here has right ascension i just said five hours of 14 minutes declination negative 8 degrees 12 minutes battle jews is north of the celestial equator so it&#39;s got a declination of plus 7 degrees 24 arc minutes and a right ascension of 5 hours 55 minutes its right ascension is bigger so it is to the east on the celestial sphere of rigel its declination is more positive so it is north of rigel on the celestial sphere so that&#39;s kind of how astronomers use directions notice that north is up south is down but east is to the left and west is to the right that&#39;s kind of the opposite of how you normally look at a map right and of course the reason why is because when you look at a map you&#39;re looking at the outside of the sphere of the earth when you look at the celestial sphere you&#39;re looking at the inside of the sphere so even though east and west are in the same directions uh on the spheres themselves if you look at the inside of the sphere east and west end up being reversed it&#39;s something that astronomers are often confused about all right so that&#39;s a lot of information about celestial coordinates uh i&#39;m going to review this in a couple of different ways but these are the the most important basic things so uh right ascension declination celestial longitude and latitude it&#39;s catalogued for all the objects in the sky and then your local system then is altitude and azimuth just the height above the horizon and the angle relative to north and we use both of these angles we both use both of these coordinate systems to measure positions of objects in the sky in observational astronomy

Transcript for:Celestial Coordinates Tutorial

Transcript for:
Celestial Coordinates Tutorial