國家地理:The Moon 月亮的故事

The Moon has always been a source of boundless fascination since ancient times. It changes its shape and location with great rapidity in the sky. It is also our closed neighbor and the only other body, besides the Earth, that humans have ever walked upon.
Whether it was believed to have been a big ball of cheese, home to the men in the moon, or possess the power to turn people into werewolves, the moon has been a perpetual source of wonder from ancient time to today. Just one quarter the size of the Earth, the moon is small compared to other more spectacular bodies in the universe. But since it's only 240 thousand miles away, a relatively short distance when compared with the vastness of space, nothing looms larger in the night sky.
Many scientists believe the moon formed about 4.6 billion years ago. One theory is that a huge asteroid struck earth with such force that rock and debris were shot into orbit around the planet like the rings of Saturn. Over time this cloud of fragments came together to form the moon. Since it has little of any atmosphere to protect it, the moon has been and continues to be bombarded by space debris. The evidence appears all over its dusty surface. Its entire surface is pocked with tens of thousands of craters. It wasn't until Galileo pointed a telescope at moon in 1609 that we got the first close look at its features.
The moon orbits the earth like the earth orbits the sun. Since the moon doesn't shine on its own but only reflects light from the sun, we see more or less of it during its monthly revolution depending on its position. We call these varying views--phases. When the moon is on the far side of the earth away from the sun, the moon is fully illuminated or full. As the moon travels around the earth, we can only see the sunlight falling on part of it resulting in crescent moons and half moons. When the moon is directly between the earth and the sun, light falls on the far side of the moon, blocked from earth's view, the moon is dark or new. It takes about 29 days for the moon to complete its cycle. On rare occasions a full moon passes through the earth's shadow and the sun's light is blocked. This is called a total lunar eclipse. The moon has quite an effect on our planet. As earth turns the moon's gravity tugs on our oceans creating the tides. The tidal forces have curved our planet's coastlines, buoyed its polar icecaps and influenced the rhythm of life. Our nearest celestial neighbor is a lot more than a beautiful view.