Alan Stahler: Sun to suffer eclipse | TheUnion.com
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Alan Stahler: Sun to suffer eclipse

The moon sweeps over Earth, July 2015, seen from a million miles away. Notice Hurricane Dolores off Baja.
Courtesy of NASA |

Dig up a spadeful of dirt, and toss it into a pile. The dirt lands pretty much where you want.

The casual way we can pile up dirt pretty much where we want it belies the precision of gravity. A pitcher releasing his fastball knows — in his head, in his arm, his hand, his fingers — precisely how gravity will tug on the ball, precisely how gravity will lead the ball to drill through the strike zone as it crosses the plate.

Gravity choreographs the stars in the “clockwork of the universe.” Before the invention of the ship’s chronometer — a clock that could keep time over the bounding main — sailors kept time by observing the gravitationally-controlled ballet of Jupiter’s moons.



Some four-and-a-half billion years ago, the solar system’s clockwork led a small planet to collide with the young Earth. The collision vaporized most of the smaller body, and a good part of the outer Earth, encircling Earth with a ring, like Saturn’s. But the ring quickly coalesced to become our moon.

Our moon has a diameter one-four-hundredth the diameter of the sun. It orbited thousands of miles closer to Earth than the sun back then, so it loomed huge — way larger than the sun — in the sky.




With Moon and Earth so close, they really felt the tug of each other’s gravity. Just as the moon raises tides in our oceans today, the moon stretched a huge mound of Earth-rock, upwards. And Earth drew up a huge mound of moon-rock upwards on the moon.

All matter exerts gravity, and the two mounds, one on Earth, one on the moon, tugged on each other. The moon’s tug on the Earth-mound acted like a brake, slowing Earth’s spin. Days grew longer. Just as we can count years in tree rings, we can count days in ocean sediments. When days were shorter, there were more of them in a year.

Earth’s tug on the moon-mound had the opposite effect — it made the moon race faster in its orbit around the Earth — it slung the moon outward.

The Apollo astronauts planted mirrors on the moon, enabling scientists to fire laser beams at the moon, and time how long it took the beam to make the round trip, earth to moon and back to Earth. Thanks to the tides — water tides and rock tides — the moon is still slipping away, roughly an inch and a half per year. (And Earth’s spin is still slowing down — one of the reasons we sometimes have a leap-second on New Year’s eve).

The moon is no longer thousands of times closer to Earth than the sun, merely four hundred times closer.

Recall that the sun is four hundred times wider than the moon. When you calculate how large sun and moon appear in the sky, the four-hundreds cancel out. Moon and sun appear the same size.

New moon was last weekend. At the next new moon — Monday, Aug. 21, just after 9 a.m., our time — the moon will begin slipping into the sun’s strike zone. As the moon covers more and more of the sun, the sun will become a crescent. Light on the landscape from the crescent sun will seem very odd.

Even a crescent sun can cause serious eye damage. Never look at the sun — even the crescent sun — without eye protection (Not ordinary sunglasses).

Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at stahler@kvmr.org


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