Full transcript
0:00okay now we're gonna take what might
0:02seem like a slight diversion for a
0:03second and look at light race so if we
0:08had an object like beaming light onto
0:12our planet say from point source a you
0:15can see the light rays going to top and
0:16a planet verse to the bottom they would
0:18diverge out they'd be coming from very
0:20different we would see them at different
0:22angles but as we get further and further
0:25away you can see the Rays get this angle
0:28here gets more and more narrow and if
0:29something was at an infinite distance
0:31the light rays coming in would be
0:33parallel lines well your book talked
0:37about it as well the sun's not an
0:39infinite distance but it's pretty darn
0:41far and so we approximate the light rays
0:44coming from our Sun as being parallel
0:46they for our purposes they really are
0:49basically parallel rays of light coming
0:51in we're gonna see this is important for
0:54a couple of things and they sent a
0:56second we're going to talk about someone
0:57who use this for historical perspectives
0:59to do some neat things will also see
1:02later in another week that it's
1:06important for telescopes as well so our
1:08Sun the light coming from it are
1:10parallel lines and that means things
1:12that even more further than that so
1:13every other star they're definitely
1:15parallel lines so we can assume light
1:18coming in to earth from these distant
1:20objects or parallel lines great well
1:25back in again 200 ish BCE Eris tossed
1:31Eratosthenes
1:32one name say Eratosthenes was a very
1:37brilliant person and he used this idea
1:40he used this idea to actually try to
1:43predict how big our planet was and he
1:47knew the earth was round was a globe and
1:51he knew that the light coming in was at
1:53parallel light beams the thing was he
1:57knew at two different locations on earth
1:59at noon at noon where that for that city
2:04when he was over here at Sian when it
2:09was noon well Sun will be coming
2:11straight down the Sun is directly
2:13her head at noon remember well he
2:16noticed that light would go right down
2:18to the very bottom oh oh well because
2:20hey again light beam was right above us
2:22but at that same time of day in a
2:27different city he noticed it wouldn't go
2:30down a well he noticed that the Sun was
2:32casting a shadow and that was because
2:34the Sun was not directly overhead at
2:38that time and from that he was able to
2:41reason out that the reason is because of
2:44the curvature of the earth and that the
2:48angle of that would be actually related
2:50to the angle of Earth so by looking at
2:54this basically using trigonometry by
2:57looking at the shadow he could actually
2:59figure out by knowing how far apart
3:02these things were these two cities he
3:05could do a ratio to actually determine
3:08how big our planet was I'm not gonna get
3:11bogged down in the actual details of the
3:13math with you too much but from a shadow
3:16he was able to actually estimate the
3:20circumference of our planet now there is
3:24debates as to how accurate he was with
3:26it he used this idea of a stadia as his
3:29are his measurement and there's parentid
3:32different types of stadia so different
3:34people say maybe it was really close
3:36maybe wasn't the end a day this is still
3:39shows you the brilliance of being able
3:42to just know us a shadow used a shadow
3:44to estimate the size of our planet
3:51now there's another weird puzzle another
3:55weird aspect we haven't referenced
3:57before we talked about the celestial
3:59sphere we take our North Pole we
4:02projected up there and we had the North
4:05celestial pole great where our North
4:08Pole on earth is looking up if you were
4:10standing at the North Pole the zenith
4:12right above you is the North celestial
4:14Pole great well we also happen to know
4:18right now that near there is Polaris
4:21which you might know as the North Star
4:22there's a star pretty close to the North
4:26celestial Pole
4:27and we call it the Northern Star because
4:29it's the one most North the most close
4:31to the North celestial Pole it's not
4:33actually lined up mind you but it's
4:34close enough that we use it as a
4:36convenient way of indicating North again
4:38remember those seafaring people they
4:40need to know directions things like the
4:42North Star would be convenient but
4:45there's a big extra piece happening
4:48so-called procession procession of our
4:52North Pole earth remember rotating on
4:57its axis we have this big imaginary
4:59spike through the earth that were
5:00rotating the planet on well what we also
5:03find is that pole where that pole is
5:07pointing to is over time procession or
5:10proced are going through precession it's
5:14changing so over the course of 26,000
5:18years where our North Pole is pointing
5:21actually is going around itself in a
5:24circle so our axis here is actually
5:28slowly going around and around in a
5:31circle so what we are looking at north
5:35most of our planet is changing over time
5:38the Stars we are looking at that aimed
5:40at if you're looking at the North Pole
5:42is changing over time so in
5:45approximately 13,000 ish years
5:47Polaris would no longer be our North
5:49Star Vega would be because we will be
5:52pointing in a very different direction
5:54so it's kind of like a top if you've
5:57ever spun a top or a dreidel yeah as it
6:00starts to slow down it starts to wobble
6:02so it spins on its axis but then as it
6:05starts to slow down again it spins on
6:08its axis or the actual top part spins
6:11too so this axis is rotating but the
6:15point that the axis is pointing at is
6:17also going through a circle as well so
6:19the motion were looking at is a little
6:22more complicated than we alluded to
6:24before now because of precession and the
6:27reason for this by the way he says
6:28actually earth is not a perfect sphere
6:30or sphere but we're actually bulged a
6:33little bit at the middle so that causes
6:36us to get a weird gravitational pull
6:39from
6:40say the Sun in the moon which causes us
6:42to have this weird precession effect