Philosophy 167: Class 1 - Part 2 Basic Astronomy: The Celestial Sphere

George Smith


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So let's start looking at this. How did we first come to have high quality evidence in any of the sciences? Interestingly enough, it comes almost entirely from the evidence in question, comes almost entirely from the skies. So what i'm gonna do from now to the break is give you an introduction to Astronomy.
After which, after the break, we'll take a 10, 12 minute break. I'll come back and show you what Ptolemy did with Astronomy. So you do not have to learn Astronomy in this course. What you have to learn is to take apart evidential reasoning. And see what's being presupposed in it and where the holes are, etc.
So you don't have to go out and look at the sky, but you can. And if you start getting interested, as many students have, always the best place to start is with Sky & Telescope. It's an extraordinary magazine. Unfortunately at the, this fall there's very little to see in the sky.
When you go out tonight and you look toward the Southwest. There are two planets visible, Mars and Saturn, but neither is very bright. Jupiter if you get up in the morning is extremely bright. Venus is visible in the morning. My daughter's telescope sits in my office. And normally I take it out in the fall and look.
And we will this fall at least to look at Saturn. Mars isn't very interesting to look at except under very high power. But Saturn is of course, he's fun to look at. Jupiter's even more fun to look at. But in the spring, I'll make sure we get to do this again, and have a chance to look at things.
All right, so look, fundamental features of Astronomy. The first thing to notice, is the stars. If you did time lapse photography, that's looking at the North Pole with a photograph going around. The stars every night form a circle. A 24 hour circle if you see the whole thing.
This could have been done very far North. The way it was thought of from time immemorial, is all of the stars which are to the best of their knowledge at the time, fixed in position with respect to one another. They're invariably called the fixed stars. They move in unison every 24 hours on one sphere.
And it's the thing that moves 24 hours. The Earth doesn't rotate is what was classically thought. People suggested the Earth rotates and the stars are stationary. The problem is where's the evidence that the Earth's rotating. That will emerge during the later part of this semester. And we start seeing evidence that the Earth itself is rotating.
But of course, the complaint was there's no sign we're rotating. We're actually rotating at God awful fast speed. When you get the actual numbers. But, at any rate. So we have a celestial sphere. We have an equator. The celestial equator sits right above the Earth's equator. They are as it were together.
The main difference between Earth's geography and celestial geography is the coordinate system around the celestial equator is not longitude and latitude. It's called declination, which is the angle above the equator and right ascension, which is given in units of time and locates things over a 24 hour period relative to one another.
For wind for example, they're gonna cross a meridian line. So we won't use right ascension and declination much, because we don't care. There are two reasons we won't use it. We don't care about the stars. Second, if you're doing observations you almost always do it in those coordinates and then convert it to the coordinates that we're gonna be interested in.
But I won't talk about it much. I'll occasionally show you declination and right ascension data, but it's just not central to this course. The other shot here is the moon descending at night, and of course every 24 hours the moon comes up, goes down as well. But if you look carefully you see the moon and Venus.
It's at I believe every eight minutes. And it's starting from the upper top. You'll notice the moon is on one side of Venus. And by the time we get to the bottom Venus has moved past the edge of the moon. Actually, Venus hasn't moved. The moon has moved.
The moon is moving from West to East, gradually completing its circuit every 27 days and so many hours, roughly 28 days. Venus takes 566 days to do the same thing, so in a single hour, the moon is going to move. I think we can almost see what it is here, around ten minutes of arc, relative to Venus, and simply is passing it.
That's the first thing to notice there. The second thing to notice is the angle. The angle's an angle relative to the arc horizon, because Venus, the moon, the sun, and all of the planets do not move along the equator at all. They move along a different line, called the ecliptic.
The ecliptic is the line the sun defines through the stars over the course of a year. The sun like the moon, Venus all of them move normally from West to East a little bit each day. Versus the stars. In the case of the sun it takes of course 365 and a quarter days to move around.
We'll get different numbers. I said 566 for Venus. That's not quite right. That's just wrong, but from time immemorial, the ecliptic, which is at a 23 degree angle, relatively to the equator has been broken into the 12 signs of the zodiac. So these go back to Babylonian days.
Here you get the Greek Latin, and English versions of them. The normal way to describe where a planet is or other object is to say which constellation it is in. And how many degrees from the edge of that constellation it is. Well of course 12 constellations 30 degrees each then we'll get these little units of 30 degrees.
North and South of the ecliptic is latitude, not totally different from geological latitude it's the, a distance North and South. No planet goes very far, like ten degrees is the greatest deviation from the ecliptic of the planets. Pluto is no longer considered a planet, so I can say that safely.
It's an exception to this, but all of the planets then known which did not include Uranus or Neptune, stay very tightly within this band. Then the longitude is the position along the ecliptic of a planet. Or, sun. Or moon. So we end up at the time with the following objects known to move around the Zodiac with each having a definite period to get back to its prior position of the Zodiac.
They are in order of their timing. The Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. Those were the objects know ,they were all known to wander around the Zodiac, and the first problem was to give an account of that motion. Questions on that before I go on?
Just as a quick remark, you're gonna be hearing all of this for the next three weeks in different versions, so don't expect to get it all tonight. I'm preparing you for the reading, but by the third week this should be fairly normal and I'm giving you an ungraded paper to help you get there after the second week.
But for the moment, all I really need to drive home is longitude, latitude along the ecliptic. The ecliptic is separate from the equator. The ecliptic is roughly 23 degrees relative to the equator. No questions on that?
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