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ASTR 101 Module 2 (2.4: The moon and planets (The appearance, size and…
ASTR 101 Module 2
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2.1 Fundamental Motions
- We have considered the various ways in which we can gather astronomical information and asked what we see in the sky
Ordinary matter (meteors, cosmic rays, lunar rocks)
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Conspicuous objects plus planets, meteors, the milky way
- Ignoring the sun and moon to start with, we have considered how the spin of the Earth affected the apparent motions of the remote “fixed” stars
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Although stars do move individually, they are so remote that the patterns do not change perceptibly for thousands of years
- We have recognized the usefulness of Polaris (the North Star) which by pure chance lies in a useful location
One: If lost in the woods at night, use the big dipper as shown to find polaris, which lies due North. This will help you sort out your directions.
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- We have finally noticed the unusual behavior of one special star: the Sun
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2.2 The Seasons
- The apparent motion of the sun, in response to the spin and orbital motion of the earth
One: Spin of the earth, rotates on its axis.
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- The importance of the tip of the earth’s spin axis in determining the Sun’s changing aspect and the cause of the seasons
On any given day the sun is just like a star, except that it’s bright and lights up the whole sky.
During our winter is acts like a southern star and follows a relatively low daily arc across the southern skies.
During our summer, it is a Northern star and climbs much higher in the sky. It drifts a little in its north-south position and over the course of time changes accumulate
- Two new physical principles
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- The slow processional changes introduced by the gravitational influence of other planets
The axis of the earth points close to Polaris, and will do so for many centuries but it does slowly change (precess)
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2.3 The size, shape and motion of the earth
- Considered the evidence that convinced the ancients that the earth is spherical and learned how to determine its size
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- Considered evidence that the earth spins on its axis (rather than the entire universe turning around us once a day)
Wrong that this happens: Earth would be torn to pieces, thrown off like stones on a slingshot, wind on our faces – ALL wrong
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Proof 3: The Coriolis Effect – Missile example. Objects launched north from the equator appears to veer right because of the earth’s axis. Explaining hurricanes; Myth that drains are different in each hemisphere
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