Full transcript
0:01so we're on to Chapter three
0:03and I feel like I should warn people
0:05this might be one of the more intensive
0:07chapters of the course it introduces
0:10quite a few terms and scientific laws
0:13that are really essential for the way we
0:15study astronomy but as a result it can
0:17be a little pretty tricky especially for
0:19people who don't have a science
0:20background there is some fairly
0:23intensive science in here and some of
0:25the more intensive formulas as well
0:27because if we're gonna have some math in
0:28this chapter so don't be afraid to pause
0:31your video don't be afraid to take some
0:33time and actually reflect on it and just
0:36make sure you're understanding it and
0:37practicing some of these ideas too and
0:39looking at some of the Supplemental
0:41resources if you need or of course
0:43reaching out contacting me and getting
0:45help as you need so just be persistent
0:48with this one even if it seems tricky
0:50okay so we had looked at the
0:55descriptions for our solar system and
0:58how we ended up getting the heliocentric
1:00model but we've still need to make some
1:03revisions one of the early observational
1:07astronomers was Tycho Brahe he didn't
1:12actually have telescopes but he made
1:13meticulous notes of the position of
1:16stars planets the Sun the moon and from
1:21those observations eventually Johan
1:23Kepler was able to derive some very
1:26important laws that describe the motion
1:28of the planets so we're gonna see he
1:32ended up coming up with three different
1:34laws and again there has to be some
1:36credit to Tycho Brahe because his
1:37observations really helped us if
1:40actually I recall correctly I believe
1:42you on Kepler actually had pretty poor
1:43eyesight so he would've been able to
1:45make these observations themselves in a
1:47way it was a very nice collusion between
1:49them nice good teamwork
1:53okay so Kepler's first law this was a
1:57crud the idea that the planets actually
2:00orbit in the shape of an ellipse and
2:02orbit just means going around the Sun
2:04the orbit of a planet is its motion
2:07around the Sun and when we had left off
2:10it was back with the Copernican model
2:12and ever
2:13still kind of felt like the perfect
2:15shape was a circle and so they still
2:17even though they finally had come to
2:19conclusion that earth revolved around
2:21the Sun they still thought it was in a
2:23perfect circle but the problem was when
2:26Johan was looking at these observations
2:29looking at the data
2:30he couldn't resolve it he couldn't
2:34resolve these to actually work properly
2:36they were still having to have those
2:37silly deference and epicycles and even
2:40then to make all of the data fit he was
2:42struggling and eventually he realized
2:44that an ellipse was the correct path
2:47looking at the orbit of Mars I believe
2:50especially he was able to figure out
2:51that all the planets orbit in an ellipse
2:53and unlit ellipse by the way is just
2:55really us stretched out circle it's
2:58longer in one area circle is has its
3:01radius the same from all sides whereas
3:04an ellipse is actually more extended out
3:06and this actually shows a great way of
3:09drawing an ellipse because an ellipse
3:10has two so-called focus points where
3:14basically if you took two pins stuck
3:16them to a piece of paper and put a
3:18string across them you could then pull
3:21that string tight with a pencil and draw
3:23all the way around to make an ellipse so
3:26the total of the distances from the to
3:29focus remain constant but it's not quite
3:32like a circle and we're gonna look at
3:36some terminology in a second but it is
3:39worth emphasizing that this is an actual
3:41more emphasized ellipse when we look at
3:44the actual paths of the planets as much
3:47as they are ellipses they're very close
3:50to being circles so okay let's define
3:54some terms we need to know how to
3:55describe an ellipse because this
3:57describes the orbit of various objects
4:01whether we're talking about a planet
4:02orbiting the Sun a moon orbiting a
4:05planet or even a satellite a human-made
4:08satellite that's put into space and
4:10orbiting say earth that would all be
4:13described with this kind of orbital path
4:16so let's look at some terms what we want
4:20to talk about is the major axis in a
4:23circle you have the diameter well kind
4:26of similar idea here
4:27is the major access it's the distance
4:29from one side to the other passing
4:31through the two focus points passing
4:34through the two focus points and one
4:36thing I forgot to mention on a previous
4:38slide but I think it was said was that
4:40the Sun is at one of the two focus
4:43points of our ellipse so like this shows
4:45planet is at a is going around in an
4:50ellipse we also can describe a few more
4:52terms we've got the major axis and the
4:55semi-major axis is half the major axis
4:58you can think of that as the average
5:00distance the planet is from the Sun so
5:04the semi-major axis given use me by the
5:07term or the letter A is the average
5:09distance the planet is from the Sun or
5:12half the major axis where major axis
5:15again this line from one side the other
5:17going through the two focal points great
5:20we can also have the eccentricity and
5:23this is kind of a weird term this
5:25eccentricity e what it is is a number
5:28it's a ratio what we do is take the
5:30distance from one focal point to the
5:32other and we divide that by the length
5:35of the major axis and basically what
5:38this is is a measure of how stretched
5:40out our ellipse is in the case of a
5:43circle the focal points would be on top
5:45of each other so our centricity would be
5:47zero basically that's a circle is an
5:51ellipse with an in Central City of zero
5:54and if we had a centricity of one that
5:57would be a straight line because it
5:58would just be the two focal points here
6:00and here you won't be able to have any
6:01give to your rope to draw the ellipse so
6:03it'll be aligned so ellipses fall
6:05somewhere in that range and if we know
6:10those two things semi-major axis
6:11eccentricity we can describe the ellipse
6:14and therefore describe the orbit of any
6:16planet if we know those two things well
6:19there's also a couple other terms to
6:21mention we have the idea of the peer
6:23heal and perihelion
6:25which is the farthest or sorry the
6:28closest with our planet gets to the Sun
6:31again this is a very like exaggerated
6:35image the Sun would not actually be this
6:38far from the other focal point
6:41and in fact the focal points tend align
6:45both within the Sun just because it's so
6:47large but perihelion our closest
6:51approach to the Sun and aphelion is the
6:54farthest I'd always think of a faraway
6:56aphelion is the farthest away we get
6:59from the Sun whereas perihelion is the
7:01closest again greatly exaggerated in
7:04this image but the idea remains the same
7:05a fee lien is the farthest away we get
7:08from the Sun perihelion the closest we
7:10get