Full transcript
0:00we're now gonna move on to chapter four
0:01which looks at the earth the moon and
0:03the sky and looks at some of the
0:05interactions between the earth and the
0:07moon and some really cool effects we get
0:08from that so we're gonna start by
0:11defining a couple terms and one being
0:14the idea of a great circle this is when
0:17you have a sphere and the sphere is just
0:19a fancy way of saying a globe earth is a
0:21sphere this in this case this image
0:23shows it cut in half but it's a severe
0:26and a great circle is one that happens
0:29to have it's a circle on a sphere and it
0:32happens to have the center of it go
0:34through the very center of the globe so
0:36something like the equator here on earth
0:39that would be a great circle and this is
0:42important because we want to find
0:43specific types of great circles so
0:46called meridians these are circles that
0:50I mean these are all imaginary circles
0:51of course one more there's not actually
0:53a big long circle going around the earth
0:55but we're imagining a circle that goes
0:58around the earth and actually passes
1:02through both poles in that case it'd be
1:05a great circle because the center would
1:07be the center of the earth but it also
1:09happens to go through the poles and we
1:11call these meridians and this is gonna
1:14be important in a second we're gonna see
1:16why because what we're gonna look at
1:20first is the idea of longitude and
1:21latitude and unfortunately these images
1:22are a bit stretched out I can see but
1:24the idea of latitude and longitude is
1:28helping us make a coordinate system on
1:30earth latitude you can see are a bunch
1:33of circles and basically latitude tells
1:36us how far north or south of the equator
1:39we are it's a number is a degrees either
1:43north or south of the Equator you can
1:45see degrees north or degrees south of
1:47the equator and it's just to help us
1:49figure out roughly where on earth we are
1:52so it gives us the up-and-down the north
1:54and south direction longitude on the
1:57other hand actually gives us the east
1:59and west direction east and west so you
2:02can see these lines are defining east
2:04and west and so between that if we have
2:07the north and south in the east and west
2:09we get a coordinate system we can
2:11describe anything on earth
2:13exact
2:13is by its describing its longitude and
2:16latitude now one thing you might have
2:20noticed right off the bat is where is
2:22the zero point for the longitude with
2:24latitude
2:25it was the equator the Equator is the
2:27greet upon zero point cuz it's very
2:29middle it makes sense but what about
2:33with longitude where do we find the zero
2:37point because actually a lot of
2:38countries wanted theirs country to have
2:41the prime meridian prime meridian is the
2:44zero point i obviously a lot of
2:46countries wanted that cuz hey why not
2:48were the center well it was eventually
2:50agreed to be in Greenwich England
2:53there's the Royal Observatory there they
2:55did a lot of work around this concept so
2:59it made sense to find that as our prime
3:01meridian the zero point so again this is
3:06giving us a coordinate system on earth
3:08and allowing us to actually figure out
3:09where anything is on earth by describing
3:12it with these two sets of coordinates it
3:14allows us to figure out exactly where
3:16things are of course may be wondering so
3:19what was that got to do with astronomy I
3:21mean most of the time we're not really
3:23looking at things on earth so great we
3:25can describe where something is on earth
3:26but I want to know where something in
3:28space is well back to the celestial
3:32sphere remember the celestial sphere
3:34this imaginary sphere around Earth where
3:37everyone thought the stars were embedded
3:39again still a useful idea we know it's
3:41horribly wrong stars are incredibly far
3:43and diverse distances from us but if we
3:47think of this celestial sphere we can
3:50actually project this coordinate system
3:52onto it remember we took the equators
3:55our the equator and projected it so we
3:58call it a celestial equator lining up
4:00with the earth's equator and we also
4:02could talk about the celestial poles
4:05lining up with Earth's north and south
4:07poles so why can't we take a similar
4:10coordinate system and project it on
4:11there of course we do add a slight
4:14wrinkle and that we change the names but
4:17the idea is still the same
4:19rather than longitude latitude now we're
4:21talking about the ideas of declination
4:23and right ascension
4:25which may sound more complicated and
4:28needs but it's still a coordinate system
4:31it's just projecting this onto the
4:34celestial sphere remember last time we
4:36left off we had the idea of
4:38constellations but that was just a very
4:41general rough direction in the sky where
4:44something was we need something precise
4:47something I can exactly tell us I mean
4:49the sky is very very very very large to
4:52say it's in a genore direction isn't
4:54gonna help us much especially if we're
4:55talking about one point light versus
4:56another it's gonna be very difficult to
4:58actually figure out what anybody is
5:00talking about unless we can very quickly
5:02narrow it down to a specific point so
5:05that's the idea of declination and right
5:07ascension so what is declination well it
5:11tells us how far north or south of the
5:14celestial equator something is hopefully
5:16sounds a little bit's million and again
5:19we're going to measure in degrees in
5:20this case the positive degrees would
5:23mean north at equator and negative would
5:26mean south of the equator so you can
5:29talk about something being at 30 degrees
5:33back in the nation that means it's 30
5:36degrees north of the Equator you might
5:38also see it sometimes written still with
5:40the degrees north or degrees south but
5:43often it's also written with negatives
5:44so if it's a negative or if it's just a
5:47positive and you're not sure what that
5:48means you've kind of safely assume it
5:50means north of the Akula lest you'll
5:52equator and right ascension this tells
5:56us how far east it or west it is and
6:00we'll talk more in a second about a
6:02specific point but talks about how far
6:04east or west it is and this can actually
6:08be measured in two ways we can talk
6:10about in degrees or we can talk about it
6:13in hours and this means actual hours
6:15because remember earth is rotating on
6:18its axis and so what we end up saying is
6:21that we can describe it as degrees or
6:23hours and the rate is 15 degrees equals
6:26one hour now where does this come from
6:28well if we take a circle is 360 degrees
6:31if we take a day it's 24 hours if you
6:35divide 360 by 24 we get 15
6:39so 15 degrees should be equal to one
6:42hour and that means when we are looking
6:45at the Earth's rotation in an hour it's
6:48a celestial sphere should rotate by
6:51about 15 degrees so that's just two
6:54different ways of describing it in terms
6:56of degrees or hours and again it's just
6:58selling how far east or west it is now
7:00what's the zero point in this case
7:02well the zero point is the sun's
7:05position during the vernal equinox and
7:08we'll look at what that is a little bit
7:10later but the vernal equinox for now I
7:11just think is a certain time of year
7:13certain time of year and where the Sun
7:15is at that time of year at noon of that
7:18day that's all that's or zero point and
7:22we're measuring east and west so what do
7:24we end up with again a coordinate system
7:26a way of describing up north and south
7:29in the east and west and so then we can
7:31map out the sky and if we want to
7:34project that onto a two-dimensional
7:35surface we can think this is the sky
7:38of course it's actually wrapped around
7:41us when we look at it but this gives us
7:44a very nice coordinate system and we can
7:46very quickly describe or something is I
7:49mean we have to go to Spiner
7:51measurements but we can actually say
7:53where exactly something is by describing
7:56its declination and right ascension
7:58gives us a coordinate system that's what
8:00this is and if this is something you're
8:02struggling with or just want to see
8:04another example you might want to check
8:06out some of the animations from our book
8:08I mean I would generally recommend some
8:10of the animations anyway or some of the
8:11extra resources I put at the end of the
8:13chapter I don't really don't repeat the
8:15ones from the book just because you can
8:17get them from the book instead but you
8:19can see this would let you actually play
8:21around with the idea of declination and
8:23right ascension and seeing how it
8:26actually moves around so it's worth
8:28checking out some of those additional
8:30resources especially if you're
8:31struggling with a concept