Answer to Question 1
Planetary motion was a big problem for ancient astronomers. In fact, the word planet comes from the Greek word for wanderer, referring to the eastward motion of the planets against the background of the fixed stars. The planets did not, however, move at a constant rate, and they could occasionally stop and move westward for a few months before resuming their eastward motion. This backward motion is called retrograde motion.
Answer to Question 2
Tides are caused by small differences in gravitational force. As the Earth and Moon orbit around each other, they attract each other gravitationally. Because the side of Earth toward the Moon is a bit closer, the Moon pulls on it more strongly and that pulls up a bulge. Also, the Moon pulls on Earth a bit more than it pulls on Earth's far side and that produces a bulge on the far side. The oceans are deeper in these bulges, and as Earth rotates and carries you into a bulge, you see the tide creeping up the beach. Because there are two bulges, there are two high tides each day, although the exact pattern of tides at any given locality depends on details such as ocean currents, the shape of the shore, and so forth.The Sun also produces tides on Earth, although they are smaller than lunar tides. At new and full moons, the lunar and solar tides add together to produce extra high and extra low tides that are called spring tides. At first- and third-quarter moons, the solar tides cancel out part of the lunar tides so that high and low tides are not extreme. These are called neap tides.
A theoretical photosynthetic response curve.
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