this is a stellarium video on procession the precession of the equinoxes and the effects of precession that you can see in the sky if you observe for a long enough period of time because the precession cycle is very slow it takes about 26,000 years for it to go through one cycle so the Earth spins on its axis once per day and so the earth has a rotational axis that actually defines the earth's equator and the celestial equator and the North and South Pole North and South celestial pole as your spins on its axis it acts like a gyroscope or act like a top now the axis is not in line with the axis of orbit around the Sun so if we watch this video here you can see that the earth is rotating to the east so counter clockwise have seen from above and that axis is tilted relative to its orbital plane which is kind of horizontal in this in this video in this animation you can actually see the axis of the orbit of the earth around the Sun is going to be along this Terminator line between night and day because the Sun has to be off exactly to the left in order to illuminate that side of the earth like so so the axis of rotation of the earth is tilted by 23 and 1/2 degrees and of course that's what causes the seasons and the variations in declination of the Sun from the summer solstice at plus 23 and a half degrees down to the winter solstice at negative 23 and 1/2 degrees so why does the earth precess around like this the two periods here and not to scale it actually since it takes 26,000 years for the axis to precess all the way around like so it would actually be many many many days of course 26,000 times 365 days that many rotations for the axis to go all the way around so the earth acts like a gyroscope and if you have a spinning gyroscope and you grab it until I try to twist it give it a torque it will not move the direction you turn it it will actually move perpendicular to that direction and of course that's how a bicycle wheel can actually balance when you try and tip it over in this case the earth is not a perfect fear because it is rotating it actually bulges at the equator and the Sun and the moon tug on the equatorial bulge making the earth try to torque so that the axis of rotation lines up with the axis of the orbit but as a gyroscope the earth doesn't turn that way it just precesses around kind of a big cone shaped circle the poles make this cone-shaped circle around once for 26,000 years so what are the effects that you can observe well one of them is that the direction that the North Pole is pointing or the direction that the South Pole is pointing is going to move around in a big circle with an angular radius of 23 and 1/2 degrees right now the North celestial pole points at with star Polaris so we have a North Star as the Earth spins on its axis that star stays fixed in a local sky but due to precession the axis shifts away from those stars maybe over here it's pointing up lares and then 13 thousand years later it's pointed over here at some other patch of sky there may or may not be a star there and so that affects whether or not there's a pole star in which star is the pole star let's take a look at that in Stellarium so here we are in New Haven it's February 2016 I want to look to the north so we can find the North Star and we'll go to speed up time here so we can get to sunset all right and that of course is Polaris right there there's a nice trick using the Big Dipper to help you find Polaris this is the handle of the Big Dipper this is the cup the two stars at the end of the cup will point at Polaris so that helps you find the North Star so I turn on the equatorial grid the lines of right Ascension and declination you can actually see exactly where there are celestial pole is where all of those lines come together so actually I can zoom in a little bit on that so you can see Polaris is not exactly right on the North celestial Pole it's about 41 arc minutes off of the North celestial pole this year now astronomers know about this and you can actually correct for that if you know what time of night it is and what season of the year it is you know that opal heiress is a little bit to the left or a little bit below the actual North celestial pole and you can adjust for that if you need to find true north to align a telescope or for whatever purpose so as the evening progresses you can see the whole sky appears to spin around in our celestial Pole and Polaris is fairly close to that famously Julius Caesar says in the Shakespeare play I am as constant as the Northern Star and actually if we were to go back in time let's stop time here and let's go back to the time of Shakespeare so let's go back to around 1600 or so Polaris was less constant then than it is now now there's about a two or three degree difference between Polaris and the North celestial Pole so if we watch this guy rotate now you can see that pilaris is making a bigger circle around the North celestial Pole there are certain seasons of the year when you wouldn't be able to see Polaris above or below very like right there just in February the Sun is up when Polaris is above the North celestial Pole I'm not too concerned about that I'm going to turn off atmosphere so that we don't we don't lose it during the daytime now if we were to actually go back to the time of Julius Caesar stop that then the North the North celestial pole is even further away from Belarus if we go back to let's say 46 BC a bad year for Caesar right now if we let time pass again and I'll mark klaris again you can see oh it's a good I'd say 15 degrees or so away from the North celestial Pole so Julius Caesar would not have actually used Polaris as a North Star you could actually perhaps used a little dipper to find the North celestial pole this is the handle of the Little Dipper here stop that and I could turn on the constellation lens for you this is a handle of the Little Dipper and then here's the cup of the Little Dipper and these two stars at the end of the cup actually point to the North celestial Pole even though there's no really really knows star there if we go back even further in time that you can find other stars for example if we go back to about 3000 or so BC you can see the star Thuban right there in the constellation of Draco is fairly close to the North celestial Pole now it's not as bright as Polaris but you can certainly see it with the naked eye of course especially before the days of light pollution we go back even further let's go back maybe thirteen thousand years the star Vega very bright star in Lyra is closer to the North celestial pole with other bright stars so maybe if you need a correction you could use that to find north but you know our cavemen ancestors perhaps used Vega to the tea route which way is north well there's no evidence of that but maybe they did that happen if we go back twenty-six thousand years from 2016 then law have made one full procession cycle and Polaris should be back close to the North celestial Pole so let's go back to roughly 24,000 BC and I'm not getting it exactly right but there you see Polaris is back close to the North celestial pole and we've gone through one full cycle another effect that you get with precession is that the position of equinoxes will change so the intersection the line of intersection between the equatorial plane there you see on the globe of the earth and the ecliptic plane which is more or less horizontal in this animation the point where they intersect is going to make a big circle in both planes all the way around once per 26,000 years so the where the equinoxes points in the on the celestial sphere is going to move around so let's actually take a look at that as well I'm going to reset back to 2016 and we're going to try and find Equinox look around here to the south leave the atmosphere offer now so let's turn on the celestial equator so we can see when the Sun is going to be on the celestial equator there is the equator let's also turn on the ecliptic line which shows us the path of the Sun around the celestial sphere so here in February February 4th the Sun is a little bit to the west of the point where it crosses the equator so we should actually cross see the Sun on the equator at declination zero at tempo I'm really at the really wrong year here is put that back right the Gregorian year isn't perfect if you go ten thousand years in the future but for now the Gregorian year is designed to stay in sync with the tropical year and keep the Sun keep the spring equinox close to March 20th and it might be a little bit before a little bit after depending on where you are in the leap year cycle but there it is right there so that point right there is where it is just because the pole was pointing off that way remember it's going to be 90 degrees relative to the equator and turn the grid back on so you can see the declination and the right ascension lines and so as the pole precesses around this point is also going to precess around let me go a couple of months ahead to get the Sun just off of that point and then I want to look at that point and just keep track of it relative to the stars now it's below the ground there at that time I'm just going to hide the ground so I'll use the hide the ground option we'd forget where it is the trees there we go and I want to orient myself so that the equator stays horizontal so there's equatorial mount mode all right and let's see so here we are now actually there's the spring equinox right there so let's zoom in on that a little bit all right and by definition we design celestial longitude the right ascension circles as being zero at that point on the celestial sphere so there you see zero hours same as zero degrees but right ascension is an hours now let's see what happens year after year so keep an eye on that point and we let time pass every so often the moon comes through and getting a there we go every solve the moons comes through and changes the contrast on the on the sky there so see how that point is moving relative to the relative to the Stars so these stars that are fairly close to the zero right ascension circle should have write a central coordinates let's see let's pick that one right there right ascension coordinates very close to zero twenty-nine hours 59 minutes 44 seconds and as each year passes you see it drifts off to the east sort of parallel to the ecliptic because the precession precession is carrying this point around to the west as the Earth's axis precesses now this is the reason why right Ascension and declination coordinates always have a year associated with them so here you say see it says right ascension declination of date and so those are the coordinates for 2080 but then also up here says right ascension declination for the year 2000 so this is how astronomers deal with the changing equinox position we tabulate the coordinates for stars or galaxies or whatever you're trying to keep track of every 50 years and put those coordinates in a catalog so we used to use the coordinates for 1950 these days we use the coordinates for 2000 and at some point we'll switch over to using the coordinates for 2050 but it's very important that you know exactly what coordinate system your telescope or your computer is using when you try to go find these objects in the sky because you could point at the wrong patch of sky just because you're not taking the precession of the equinoxes into account now in terms of other things you can observe what what this means is that where the Sun is relative to the stars on particular points in the seasonal year the spring equinox the summer solstice the fall equinox and so forth it's going to be in front of different stars as the precession cycle happens this is why people Sun signs don't really line up the way they're supposed to anymore for example I was born in August little my birthday here and I'm my Sun Sign is supposedly a Leo and so if I were to go find okay where is the Sun today there's the Sun over there and it looks like the Sun is actually in the constellation of cancer although it's not too far away from Leo it's kind of midway between cancer and Leo that could come near the end of the the time range for Leo well that's because the starting point when astrology was established thousands of years ago was in Aries so the spring equinox happened in Aries so if we look at where the spring equinox is today what constellation is it in it's sort of in Pisces although it's kind of halfway between Pisces and Aquarius but if we go back go back a thousand years you can see there it's more solidly in Pisces we go back 2,000 years you can see yet still still pretty much in Pisces we go back 3,000 years negative 1,000 you can see now here it is in the constellation of Aries Aries is a very dim constellation but it gets its own 30 degrees of the ecliptic kind of right around here we go back 5,000 years well to 5000 BC so 7000 years you can see here it is over here in Gemini so in the spring the Sun would have been in Gemini and the constellations you'd see for example rising opposite the Sun would shift every every two thousand years they shift up by one zodiac sign another example of this is that two useful rule of thumb to know that when the Sun sets on the winter solstice let's go back to 2016 when the Sun sets on the winter solstice the constellation of Orion is rising in the West so let's go to the winter solstice December 21st more or less and we'll go back to the horizon mode because I want to see the Sun setting turn on the ground we'll go back to orient to the horizon and then I want to try and find sunsets all right there's the Sun up in the sky turn the atmosphere back on and let's go to sunsets all right so as the Sun sets in the west on the day of the winter solstice you have the constellation of Orion rising in the east so what stars are behind the Sun or opposite the Sun 90 degrees to the Sun will shift with precession in terms of the seasonal year so if we go back a thousand years there's been was higher in the sky at sunset we go back 2,000 years it's even higher at sunset to go back a good chunk of time let's even go back that seven thousand years I did before there you see Orion is quite far from the celestial equator to kind of creep down to the south because of the way that the equinoxes have processed around and so it's no longer a good marker for the winter solstice this is the peril of using a side aerial cycle whereas the Sun relative to the Stars to track a tropical cycle or a seasonal cycle because a precession will very slowly get out of sync so for example let's say you had a temple or an architectural alignment that you thought aligned with a particular star because it aligns with a particular azimuth maybe where the bright star Sirius rises or the bright star Canopus that will actually shift the direction where that star rises on the horizon will shift over a 26,000 year cycle now you can actually use that potentially to date observations if you have evidence that a star was supposed to rise at a particular altitude a particular azimuth you could actually use precession to help to you date when would that have happened at a particular latitude now it's an interesting question to see if precession would affect the Solstice alignments so we've talked about Stonehenge and how the main axis of Stonehenge appears to align with the direction of the summer solstice sunrise and then a hundred eighty degrees in the other direction it aligns with the winter solstice sunset now Stonehenge is very old the layout of the stones maybe go back to two thousand five hundred or so BC but the alignment with the Solstice is probably much much older perhaps going back five sixty seven thousand BC or so so with that alignment break due to precession well it's important to remember that the Solstice directions are determined by the extremes of declination of the Sun from minus twenty three and a half the plus twenty three and a half and the latitude of your site so if the latitude of Stonehenge doesn't change would the extremes of declination of the Sun change so so when the sun is shining more directly on the southern hemisphere that's going to be our winter solstice and also for Stonehenge when the sun's shining more directly in Northern Hemisphere that'll be our summer solstice the only reason those declination x' plus or minus twenty three and a half would change would be if the degree of tilt change if the earth had less tilt or if the earth and more tilt the earth had left tilt we'd have less extreme seasons and the variation of declination of the Sun would be less we had more tilt tilt we'd have more extreme seasons and the Sun might go plus or minus thirty degrees or 45 degrees or sixty degrees we had a bigger tilt the earth's tilt the degree of tilt the oblique wa t does change very slowly that's an effect called mutation over a much longer time span or roughly a hundred thousand years instead of twenty-six thousand years so that should not affect the Solstice alignments and let's just check that in Stellarium quickly so let's go back to 2016 and let's go to see where the Sun rises on the summer solstice so here we go and so let's go look in the east and find a sunrise I don't have a Stonehenge circle set up in there sir Eisen it would actually be possible in Stellarium to upload a custom horizon that would have the stones as you see them from the center of the circle what do I have what might serve as a front sight here for my solar observatory well if I let the Sun cross the horizon here you can see yeah there's kind of a notch in the horizon image background and I could use that as a marker to see if the Sun is going to rise at that point it's important to check and make sure that that really is an extreme northern declination of the Sun I can just check that here and see oh yes 23 degrees 26 minutes I could also you know step forward a couple days or backwards a couple days and make sure that the Sun really is at an extreme there if we go back a thousand years and look and see okay well where's this I'm going to rise rising it pretty much the same as Ana we go back 2,000 years again sun's rising at the same azimuth on the Solstice we go back to the time of Stonehenge let's go back to 2500 BC alright now it looks like it's at a different out as a method a different asthma so maybe it's broken well this is the Gregorian calendar and the regarding calendar does actually lose days relative to the tropical cycle so I'm going to actually kind of fish around and make sure that I'm at an extreme point of declination that looks like that looks pretty good right about there you can see the Sun was in Leo in the direction of Leo in 2500 BC so instead of having the Tropic of Cancer we'd have a tropic of leo all right now where is the Sun rising at what as the most of that same azimuth right because the extremes of declination of the Sun haven't changed very much maybe it's changed by a fraction of a degree due to notation but not enough to affect the precision of these alignments so solstice alignment seemed to be fairly robust over thousands of years but directions to where stars rise will change significantly over a few thousand years because of the precession effect