Here are 10 cool, and sometimes surprising, facts about Earth's nearest neighbor: the moon.
FIRST STOP: How the Moon was Made
NEXT: Locked in Orbit
Long ago, the Earth's gravitational effects slowed the moon's rotation about its axis. Once the moon's rotation slowed enough to match its orbital period (the time it takes the moon to go around Earth) the effect stabilized. Many of the moons around other planets behave similarly.
What about phases? Here's how they work: As the moon orbits Earth, it spends part of its time between us and the Sun, and the lighted half faces away from us. This is called a new moon. (So there's no such thing as a "dark side of the moon," just a side that we never see.)
As the moon swings around on its orbit, a thin sliver of reflected sunlight is seen on Earth as a crescent moon. Once the Moon is opposite the Sun, it becomes fully lit from our view — a full moon.
Later, the seeds were germinated on Earth, planted at various sites around the country, and came to be called the moon trees. Most of them are doing just fine.
More Than One Moon?
Cruithne, as it is called, takes 770 years to complete a horseshoe-shaped orbit around Earth, the scientists say, and it will remain in a suspended state around Earth for at least 5,000 years.
NEXT: A Cosmic Punching Bag
The scars of this war, seen as craters, have not eroded much for two main reasons: The moon is not geologically very active, so earthquakes, volcanoes and mountain-building don't destroy the landscape as they do on Earth; and with virtually no atmosphere there is no wind or rain, so very little surface erosion occurs.
The Moon is an Egghead
NEXT: The Moon Shakes
Scientists say they think the moon probably has a core that is hot and perhaps partially molten, as is Earth's core. But data from NASA's Lunar Prospector spacecraft showed in 1999 that the moon's core is small — probably between 2 percent and 4 percent of its mass. This is tiny compared with Earth, in which the iron core makes up about 30 percent of the planet's mass.
NEXT: Is the Moon a Planet?
NEXT: Origin of Earth's Tides
The moon's gravity pulls on Earth's oceans. High tide aligns with the Moon as Earth spins underneath. Another high tide occurs on the opposite side of the planet because gravity pulls Earth toward the moon more than it pulls the water.
At full moon and new moon, the Sun, Earth and moon are lined up, producing the higher than normal tides (called spring tides, for the way they spring up). When the moon is at first or last quarter, smallerneap tides form. The Moon's 29.5-day orbit around Earth is not quite circular. When the moon is closest to Earth (called its perigee), spring tides are even higher, and they're called perigean spring tides.
All this tugging has another interesting effect: Some of Earth's rotational energy is stolen by the moon, causing our planet to slow down by about 1.5 milliseconds every century.
NEXT: One Day, the Moon Will Leave