Full transcript
0:00we're now gonna move on to chapter 2 but
0:02only section 1
0:03remember we might cover some chapters
0:06out of order and in this case we're
0:07actually gonna cover the rest of chapter
0:092 next week but we're looking at section
0:122.1 because it's all about the sky above
0:14us actually looking at the sky and
0:16seeing some of the early conclusions
0:18people drew and some of the early ideas
0:20that might still be useful for us now
0:24but we're gonna have to keep in mind
0:26that early on humanity had a very
0:30geocentric or Earth centered view of the
0:33universe we quite literally thought that
0:36the earth was the center of the universe
0:37we thought we were very incredibly
0:39special humanity was great and special
0:42and on earth and earth must be the
0:44center of all things and we're gonna see
0:46in a second it kind of made sense when
0:48you look to the sky to have this idea
0:51but first let's look at a few terms if
0:54we were to go out and look at the night
0:56sky and if we say had no buildings in
1:00the way and could look in all directions
1:01what we'd have is a clear view of the
1:05night sky and right above us is what's
1:08defined as the zenith so if you look
1:11right above you in the sky that's the
1:13zenith and where the sky would meet the
1:16ground or seem to at least from our
1:18perspective is defined as a horizon so
1:21that's as far as you can see the horizon
1:23the horizontal point where you can no
1:26longer see the rest of the sky because
1:28it follows below the horizon stars are
1:30still there but we can't see them
1:32because they're out of you
1:34there's the land blocking away but early
1:39on there is some theories that if you
1:42looked at the night sky you saw all
1:44these stars and people actually thought
1:46they were projected that they were
1:47basically embedded onto a celestial
1:49sphere a celestial sphere that was
1:53rotating around the earth much like this
1:55image shows you see it looks like this
1:57big globe surrounding Earth and some
2:00early humanities actually thought that
2:04it was the stars embedded on the sphere
2:07and this is what was rotating around
2:09Earth and again when you go out and look
2:12at the night sky
2:13to make some sense if you watch it over
2:16a long period of time it would look like
2:18the stars are kind of rotating around us
2:20and from our perspective it usually
2:24looks like the stars are staying at a
2:25fixed distance relative to each other as
2:28well so it kind of seems like they're
2:30embedded we now know if we actually look
2:32with very proper telescopes that the
2:34stars don't stay a fixed distance but
2:37they didn't have those kind of
2:38techniques they didn't have the
2:39extensive records we have now so it
2:42looked like the stars were embedded on
2:44this big celestial sphere surrounding
2:46earth and rotating around us and we're
2:49gonna see later on that this idea is
2:51still useful it's not right obviously we
2:53now know that stars are actually way out
2:56in the sky and at various distances from
2:58us but we can still talk about the
3:00celestial sphere as a useful idea a
3:02useful way of describing things so if
3:07we're gonna use it we got to find a few
3:08more terms well
3:10the celestial sphere can have the same
3:13kind of coordinates or same important
3:16points as Earth so Earth has a North
3:19Pole and a South Pole well we can define
3:21a North celestial Pole and a South
3:23celestial pole and they are just the
3:26North Pole of Earth projected on to this
3:29imaginary sphere just like this image
3:31shows if we go straight up from the
3:34North Pole if you were standing on the
3:35North Pole the zenith would be the North
3:38celestial Pole and the same with the
3:41South Pole and we can also define a
3:43celestial equator which is exactly lined
3:46up with the earth's equator it's the
3:49earth's equator projected out onto this
3:51celestial sphere again that's all
3:53imaginary there's no actual celestial
3:55sphere but it will be useful later great
4:01well how we see the Stars is actually
4:04going to depend quite heavily on where
4:06we are on earth because again remember
4:09celestial sphere is as if the stars are
4:12rotating around us on this big giant
4:13sphere and if you were say this person
4:16right here in this image you'd see the
4:17stars rotating past us in the night sky
4:20but what would happen if you were at the
4:23poles
4:24this is an actual time-lapse photography
4:26or if
4:27time lapse photo of the poll and you can
4:30see it's I mean for one thing it's kind
4:33of a spectacular image but it's the
4:34stars are being rotated over time but at
4:38the polls it won't look like they they
4:41won't go out of you you will look up and
4:43see them as seeming to go in a circle
4:45around you throughout the night it's as
4:48if that pole you're looking up and
4:51actually seeing the stars just rotate
4:52around because we thought again / - a
4:54sphere they're meted on there so at the
4:57polls is must be where they're rotating
4:59we now know all of this again is of
5:01course because earth is actually
5:03rotating and our axis of rotation which
5:06the axis of rotation is a fancy way of
5:08saying the line we rotate about it would
5:12be as if you took a big metal spike
5:14stabbed it through earth and we're
5:16rotating earth on this metal spike that
5:19is the axis of rotation the big spike
5:22stopping through earth that is caught
5:24over rotating on well it happens to go
5:27through the poles so when we look up
5:30from the poles we don't see the Stars
5:32disappear out of view
5:34we just see them seemingly rotate about
5:36the poles in these big circles whereas
5:39if we were back at this point here we'd
5:42actually see the stars come in and out
5:43of view
5:46now some weird complications that arose
5:49were the Sun for one thing the moon they
5:53seem to move relative to the Stars and
5:56we're gonna focus just on the Sun for
5:57now but the Sun would over time seem to
6:01move relative to the stars and if you
6:03tracked it outdated north sorry night
6:05tonight we would actually see it at a
6:08different point relative to stars so it
6:10seemed to be rotating about us not just
6:14on a celestial sphere and what they
6:17actually defined is this idea of an
6:20ecliptic the ecliptic seemed to be the
6:23path the Sun takes around us now again
6:26we now know we actually rotate around
6:29the Sun and it's because of our rotation
6:32that the Sun seems to go in and out of
6:34you throughout the day
6:35but again we can think of this lesser
6:37sphere as a useful point one thing here
6:41no
6:41right off the bat though this so-called
6:43ecliptic this path the Sun seems to take
6:46around us is actually tilted relative to
6:50our equator it's actually tilted at an
6:53angle of about 23 and a half degrees now
6:56might be wondering why the reason is is
6:59that earth is tilted that axis of
7:02rotation the big metal spike
7:04stabbing through earth in rotating us
7:06that's tilted 23.5 degrees relative to
7:11the path we take around the Sun and this
7:14is something we're gonna build on in
7:15Chapter four a bit but the idea will be
7:18important but just keep that in mind we
7:20are actually tilted our path at about
7:2223.5 and thus the Sun if we're looking
7:25at it in terms of a celestial sphere
7:27where the Sun seems to be rotating
7:29around us then it is on a tilted orbit