00:01
So here we'll talk about what you can see in the night sky and how these objects in our sky move if you're standing on the earth, looking up at them.
00:11
So, of course, the most common thing that you see in the night sky, at least thousands of years ago, this would have been the most common are the stars.
00:22
And from the northern hemisphere, what those stars do in motion is a little bit different depending on whether, you're facing north or facing south.
00:37
Facing north versus facing south.
00:40
But the basic idea is if you're facing north, some of the stars do what's called circumpolar motion.
00:49
So there's a point in our sky from the northern hemisphere that does not appear to move.
00:56
And the stars, so supposed to be little stars, actually go and, big circles around that point.
01:07
This is called circumstellar motion.
01:13
But as the star gets closer and further and further away from that pole, what you see happen is the star basically rises up out of the east and sets in the west over a 24 -hour period.
01:41
Similarly, let's say that you're facing south instead.
01:45
So here's your horizon, and east and west are reversed, if that is true.
01:53
But what isn't reversed is you will still see stars rise up out of the east and set somewhere over in the west.
02:03
And they'll go up over your head and eventually over time set in the west.
02:09
But we do know that the circle, if you get to see the full circle of the star, it takes 24 hours or one day to make a full circle.
02:21
So our time of one day is based on this motion.
02:28
And we won't get too complicated there.
02:30
The other object that you can view, of course, is the moon.
02:35
And if we will look at the moon from the northern hemisphere, so not too surprisingly is that if the moon does rise, now sometimes the moon is not visible during the night.
02:51
But if it does rise, it comes up out of the east and it makes an arc through the sky and sets over in the west at some point in time.
03:06
And of course, the time that it rises and sets depends on its phase.
03:10
We won't get too much into that.
03:13
But it rises east, sets west on a nightly basis.
03:23
On a much longer term, though, the moon actually moves against the backdrop of stars.
03:31
And this was known to people for a very long time that if you watch the moon over a long period of time, it actually moves the opposite way against the backdrop of stars.
03:46
So you can go out, okay, let's draw some stars in the moon.
03:51
Let's draw a little cluster of sars.
03:57
And let's say you viewed that on, i don't know, monday night.
04:07
So the moon is a little bit west of that, say, 8 p .m.
04:17
And if you came back a couple nights later, what you would notice is that the moon would have actually appear closer in the sky.
04:29
To that group of stars, say wednesday at 8 p .m.
04:38
So over a long time, a long time period, and we're really talking about a month, the moon goes west to east, and this is against the backdrop of stars.
05:04
And this led many early people to realize that the moon was way closer to the earth, that the stars were hard to figure that out, but it's kind of like looking at two people moving, one at a close distance and one at a far distance, and that closer person will appear to move against the backdrop of faraway objects or people, etc...