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quote:According to the Earth Data box, Earth rotates once on its axis (relative to the stars) in 23h 56m—1 sidereal day. However, we know from fossil measurements that Earth's rotation is gradually slowing down, causing the length of the day to increase by about 1.5 milliseconds (ms) every century—not much on the scale of a human lifetime, but over millions of years, this steady slowing of Earth's spin adds up. At this rate, half a billion years ago, the day was just over 22 hours long and the year contained 397 days. A number of natural biological clocks lead us to the conclusion that Earth's spin rate is decreasing. For example, each day a growth mark is deposited on a certain type of coral in the reefs off the Bahamas. These growth marks are similar to the annual rings found in tree trunks, except that in the case of coral, the marks are made daily, in response to the day—night cycle of solar illumination. However, they also show yearly variations as the coral's growth responds to Earth's seasonal changes, allowing us to perceive annual cycles. Coral growing today shows 365 marks per year, but ancient coral shows many more growth deposits per year. Fossilized reefs that are five hundred million years old contain coral with nearly 400 deposits per year of growth. Why is Earth's spin slowing? The main reason is the tidal effect of the Moon. In reality, the tidal bulge raised in Earth by the Moon does not point directly at the Moon, as was shown in Figure 7.22. Instead, because of the effects of friction, both between the crust and the oceans and within Earth itself, Earth's rotation tends to drag the tidal bulge around with it, causing the bulge to be displaced by a small angle from the Earth—Moon line, in the same direction as Earth's spin (Figure 7.24). The net effect of the Moon's gravitational pull on this slightly offset bulge is to reduce our planet's rotation rate. At the same time, the Moon is spiraling slowly away from Earth, increasing its average distance from our planet by about 4 cm per year. This process will continue until Earth rotates on its axis at exactly the same rate as the Moon orbits Earth. At that time the Moon will always be above the same point on Earth and will no longer lag behind the bulge it raises. Earth's rotation period will be 47 of our present days, and the distance to the Moon will be 550,000 km (about 43 percent greater than at present). However, this will take a very long time—many billions of years—to occur.
quote:Originally posted by tony6789i thought the earth's spin increasedNEVER! underestimate youth