Greek views of the earth and the stars [1.2]
In the first part of the class we'll study some astronomy
- Solar radiation, trapped by our atmosphere, is what heats Earth and keeps us from freezing out in cold space.
- The basis of climate / seasons / differences between equator and poles
- An example of a scientific model that has evolved over centuries
- It's kinda fun!
In our textbok
- Go ahead and read Chapter 1 in Hobson's textbook!
- Write down in your notes or notebook answers to his "Concept Check" questions as you go
Stars in motion
The Greeks could see (easier than us) the stars at night, without telescopes.
- What do you notice?
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[What do you already know...] What is a constellation?
- Are constellations just temporary? Do stars leave / enter a constellation over the course of a night? a year?
- What do you wonder about?
- Stars do not come and go from constellations. The constellations are constant groups of stars.
- $\rightarrow$ The stars don't (appear to) move relative to each other. They move together across the sky in the course of one night.
- The stars in the video are rising up from the horizon. So, the video is probably looking East because that's where our Sun rises every morning.
The Greeks thought that the motion of the stars happened because they are all stuck to a "crystal sphere" which turns around Earth.
But nowadays we think that what we see as the "motion" of the stars is actually happening because Earth is rotating. This idea provides a tidy explanation for why stars *and* the sun both rise in the East (and set in the West).
Point a camera North. Now raise it up a bit to look towards the sky. Now leave the lens open a few hours, and you'll see this pattern of star motion in the heavens above. (See also Unsplash: star circles.)
Jeffrey Lovelace - "Stop Turning"
Which way are they moving?
[Writing - Rotating Stars]
What else do you notice about the motions of the stars?
We decided that from the point of view of this picture (looking North), the sun rises in the East and sets West, so the Sun would look like it's circling CCW (counter-clockwise), as would the stars, also circling around the north star.
But, nowadays, we think it's the Earth that's moving, so Earth would be rotating the opposite of the apparent star motion: CW (clockwise) as we look towards the North Star.
Here we're looking North, and slightly up at the North Star.
Now let's execute another flip around and look down on Earth from the point of view of the North Star. We see the top (North Pole) of Earth, which appears to be rotating CCW.
Maybe this image (view of the North pole "from above") makes it easier to think about the Earth's rotation...
Goshen is pictured as a red dot, near Lake Michigan.
- Imagine now that the Sun is sitting still, and Earth is rotating, one way or another.
- From Goshen, looking towards the sun... Are you looking East (towards New York) or West (towards LA)?
- So is this snapshot of Goshen at sunrise (and we'd be moving into the sunlit realm) or sunset (and we'd be moving into the night-time / dark realm)...?
- So, what does that imply about which way Earth appears to be rotating from above the N Pole?
Here's a timelapse, but is it going right?
The Greeks viewed the stars as attached to a crystal sphere that encloses Earth, and rotates around Earth. We might call this a model. How good a job does it do of reproducing the observations?
What about the moon?
Is the moon also part of a constellation?
What does it look like over the course of one night?
Video of 'moonrise'
But if you wait for a week and watch again:
- The moon still looks like it's moving together with all the stars,
- But it's in front of a different group of stars than it was before!
If the moon is not always a part of the same constellation, how do you make sense of the video?
Aside from the moon, there are 6 other objects that you can see with the naked eye that behave similarly...
5 of thes objects, called "the wanderers" by the Greeks are named Mercury, Venus, Mars, Saturn, and Jupiter.
The planets.
The 6th one is the Sun.
It is important to remember that the planets do not appear to behave noticeably different from the stars on any given night. It is only when you watch them over the course of several days (moon / sun) or weeks or months (planets) that you might start to notice that they've "wandered off" a bit, and have not stayed together with the same stars.
Earth-centered Universe
The ancient Greeks described the universe as a series of concentric shells centered on Earth.
Well, except for Aristarchus...
Aristarchus (approx.
310 BC - 230 BC) prSoposed a sun-centered universe, in which the Earth rotated(!) and moved around the sun.
But his model was not taken seriously...
BTW, Aristarchus also successfully calculated the distance to the moon.
The planets (the 'wanderers')
Unlike the sun and the moon which move uniformly (constant speed) relative to the 'background' of stars, the planets move erratically.
Their motion is now faster, now slower, and sometimes even backwards ('retrograde').
They also get brighter and dimmer.
Epicyles
Ptolemy wrote up the prevailing Greek view in his Almagest (ca. 150
A.D.), which managed pretty good agreement with observations made at the time.
Telescopes were unknown until Galileo. The Greeks made measurements by sighting along rods.
Exercises in Chapter 1
Concept Checks in Chapter 1: 1-6, using this picture for #5 and #6.
Conceptual Exercises (CE) in Chapter 1: 1, 4, 6
