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December 7, 2000
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Moon Dance
| The Tide Is High |
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While it seems as if the Moon is a pretty constant fixture, it actually has been quite active since its birth some 4 billion years ago. Scientists believe that the Moon was initially located a mere 10,000 miles away from Earth. At that time, the Moon's gravitational pull caused Earth to spin around much more quickly than it does today, providing days just 10 hours long. Over time, the Moon began moving away from Earth. Because the Moon's gravity was less powerful, the Earth's rotation began to slow. By the time of the dinosaurs, a day stretched to 22 hours. The Moon is now about 240,000 miles away from Earth, and our days contain 24 hours. Although the Moon measures little more than a quarter of the diameter of Earth (think of the difference between a basketball and a tennis ball), the Moon's gravity still has a powerful effect on our oceans, giving us two high and two low tides per day. Newton's Law of Universal Gravitation explains this by stating that everything is pulled by the gravity of another object, and everything exerts its own gravitational pull. The Moon's gravitational pull causes the ocean to bulge towards it in high tide whenever the Moon is directly overhead. At the same time, the ocean on the opposite side of Earth also bulges, due to centrifugal and other forces. (Centrifugal force is what you feel when you're riding in a car that takes a corner very quickly.) As Earth spins, each ocean finds itself beneath the Moon about every 24 hours. On most coasts in North America, the difference between high and low tides is several feet. In Nova Scotia's Bay of Fundy, however, the difference between tides is 53 feet!
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The Moon is a familiar sight in our night sky. This somewhat ghostly looking celestial body affects life on our planet in a variety of ways. How can something a quarter of the size of Earth affect our lives so much?
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| Long-Distance Relationship |
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The Moon's pull on our oceans causes Earth's rotation to slow about 1.5 milliseconds per century. This same dynamic causes the Moon to rise into a higher orbit of Earth—by about 3.8 centimeters (almost 1.5 inches) per year.
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| Suddenly Satellite |
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For many years, scientists argued about what caused the Moon to form. The most recent and widely held view, developed in 1975, is called the Impact Theory.
Scientists now believe that some 4 billion years ago, a planet the size of Mars collided with our young Earth. At the time, Earth contained no oceans, just active volcanoes. After impact, fragments of the wayward planet exploded into the atmosphere along with some of Earth's crust. Rocky debris from the crash began circling Earth, held in place by the pull of Earth's gravity. Slowly, the fragments began massing together into one clump. That clump became our Moon, the Earth's only natural satellite. |
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