all right one more law one more law however before we do law number three okay before we get to law number three there's one thing i need to talk about okay there's one definition that we're going to need okay and that definition uh is for a special unit in astronomy okay so check this out what's a typical distance unit give me any distance unit don't worry about space don't worry about space like if i wanted to know how far apart my hands were what's a distance unit i could use to talk about well miles is a good one yeah miles is a good one feet centimeters meters kilometers yards those are all good distance units to use in everyday life right but why are miles and kilometers even not a good thing to use for distances between the planets why is it going to be a hard thing to do to use miles or kilometers uh for distances between planets yeah they're way too far apart right they're way too far apart um does anybody know how many miles the sun is away the sun's actually not considered to be that far away in astronomical terms but does anybody know how many miles the sun is away sometimes people know it i don't really expect anybody to nah it's okay it's about it's about uh 93 million miles right the sun is 93 million miles away and we're not the closest planet to the sun there's planets that are 20 30 times further out okay so if we want to use miles or kilometers we would have to use giant numbers all the time and we don't like to do that so in astronomy we actually came up with a few different units uh for talking about distances in space uh and i i'm gonna go ahead and talk about two of them real quick two different units we use in space now the first one uh has a super boring name it's just called the astronomical unit okay the astronomer what's a unit we use in astronomy the astronomical unit let me write that down up here astronomical unit which is usually abbreviated as a u capital a capital u astronomical unit so this is a much bigger unit than a mile that we use for talking about distances between the planets um but units have to have something as their standard okay something has to be the standard in other words it has to be defined as something okay and so what is the standard for distances between planets well we picked one of the planets to be the the distance between that planet and the sun was the standard for astronomical units anybody want to take a guess what planet we chose as the standard planet take a guess which planets the standard planet we picked earth we picked earth why did we pick earth why would we pick earth over other planets pretty simple reason yeah because we live there we live there and yeah humans are kind of self-centered so here we go uh one astronomical unit is the average distance between the earth and the sun okay one astronomical unit is the average distance between the earth and the sun right um why do i have to say average why can't i say an astronomical unit is just the distance between the earth and the sun why do i have to define it as the average distance between the earth and the sun why is that word average gotta be in there yeah like we were talking about in law number one and law number two planets are sometimes closer and sometimes farther away so we take the average to be the standard for the unit and that means that instead of saying that the earth is 93 million miles away from the sun we can just say that the earth is one astronomical unit away from the sun and that's a much easier number to to deal with and the planets that are closer to the sun um like mercury uh mercury is about 0.4 a.u mercury is closer to the sun so it has you know like 0.4 a.u that's not a hard number to understand does anybody know which planet is farthest away from the sun which planet is farthest away from the sun remember not pluto because pluto's trash right what planet's farthest away from the sun don't worry if you get it wrong it's okay neptune neptune it's neptune so neptune the farthest planet from the sun is on average 30 a.u whoa 38 there you go 30 a.u okay let me restart this um so that's a much easier number to deal with uh than these giant ridiculous amounts of miles that's the farthest planet away from the sun so the biggest number we're going to deal with with planets is just 30. way easier to understand um there are a few things that are farther out than than the uh than the planets in our solar system there's a bunch of uh comets that go much further out but typically you don't get things orbiting around our sun much further than about a hundred au so the biggest number that you have to deal with is just a hundred okay now here's the uh one other thing i gotta say here though what would happen if i tried to use astronomical units to tell the distance to the closest star remember what i was talking about the uh the space between the stars uh a couple of other times would i want to use astronomical units to do distances between stars no why not why would i not want to use astronomical units to tell distances between the stars we run into a similar problem that we talked about a little while ago yeah i think you guys are getting it some people are trying to give you just a second here to give that a chance stars are actually too far apart okay so just like it was hard to use miles for distances between the planets so we actually had to come up with astronomical units to have easier numbers to work with um the closest star uh to us is over 60 000 astronomical units away and that's just the closest one most of the stars out there are millions and millions of astronomical units away so uh if we tried to use this for that we would actually run into the same problem where we uh we can't we don't like to deal with these giant numbers right so instead we came up with another unit and this one's very popular you probably have heard of it before um does anybody know what the other main unit that's used in astronomy is called i think you guys probably have heard of it what's another unit that's used in astronomy yeah light years light years so we're not going to see light years for a while but i just wanted to go ahead and get it out right here so check it out light years what is a light year well it's it's there's a bit of a problem with the with the word light year um the word year is a unit of time so a lot of times in sci-fi movies they'll use light years as a unit of time like they'll have an alien show up and he'll be like oh my age is 45 light years i'm 45 light years old that doesn't make sense this is a distance unit like meters or miles that would be like a human being saying oh my age how old am i my age is 45 miles i'm 45 miles old right that's that's that's yeah they do that in sci-fi movies all the time but yeah so this is a much bigger unit um we use astronomical units for distances between planets or things in a solar system and we use light years for distances between stars uh and galaxies and anything bigger like that okay so what is a light year um well it has something to do with a year and something to do with light um and it has something to do with a special property of light that we haven't talked about yet check this out special property of light um there's something very special about light speed okay now a lot of people know that lightspeed is very very very very fast but that's not the most special thing about it does anybody know what's special about light speed it's a very important property of it that's okay i'll just tell you guys it's constant okay all light goes the exact same speed no matter what okay all light goes the exact same speed no matter what so if we had like a light race like if you and me had like a flashlight and we were just and just shot some light out it would go the exact same speed and we would have a tide light race and so you guys are getting this here what a light year is is the distance light travels in a year okay the distance light travels in a year is a light year i don't actually have to say average that time why don't i have to say average i can just say the distance why can't i why don't why don't i have to say average why can't i just say the distance light travels in a year yeah it's because of that constant speed because the speed of light is constant every single time light will go a light year in a year so how big is this um does anybody know how long it takes light to get to us from the sun that's actually sometimes people know that fact it's it's uh does anybody know how long it takes light to get from the sun to us this is light coming to us from the sun uh okay nobody's guessing this time it's eight minutes okay even though light is ridiculously fast it can travel 93 million miles in eight minutes takes eight minutes uh to get to us from the sun oh so some of you guys guessed roughly right right now check this out if light can go 93 million miles in eight minutes we know a light year is bigger than an astronomical unit but is a light year a lot bigger than an astronomical unit or just a little bit bigger than an astronomical unit is the light year a lot bigger or a little bit bigger it's a lot bigger okay it's a lot bigger actually because check it out i just said that the light can go an astronomical unit in eight minutes because it gets it takes eight minutes to go to us from the sun right and uh if it takes eight minutes to go an astronomical unit a light year must be very very very very big okay so keep that in mind if we want to talk about distances inside the solar system we use this and outside the solar system we use this one light year that's l y okay