How did the moon form? Why do we always see only one side of it? Why does the lunar day last one Earth month? Scroll down for the answers—and other facts about our moon.
• How did the moon form? According to the "giant impact" theory, the young Earth had no moon. At some point in Earth's early history, a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. Instantly, most of the rogue body and a sizable chunk of Earth were vaporized. The cloud rose to above 13,700 miles (22,000 kilometers) altitude, where it condensed into innumerable solid particles that orbited the Earth as they aggregated into ever larger moonlets, which eventually combined to form the moon.
• By measuring the ages of lunar rocks, we know that the moon is about 4.6 billion years old, or about the same age as Earth.
• The distance between the Earth and its moon averages about 238,900 miles (384,000 kilometers). The diameter of the moon is 2,160 miles (3,476 kilometers). The moon's mass—the amount of material that makes up the moon—is about one-eightieth of the Earth's mass.
• Because the force of gravity at the surface of an object is the result of the object's mass and size, the surface gravity of the moon is only one-sixth that of the Earth. The force gravity exerts on a person determines the person's weight. Even though your mass would be the same on Earth and the moon, if you weigh 132 pounds (60 kilograms) on Earth, you would weigh about 22 pounds (10 kilograms) on the moon.
• The rotation of the moon—the time it takes to spin once around on its own axis—takes the same amount of time as the moon takes to complete one orbit of the Earth, about 27.3 days. This means the moon's rotation is synchronized in a way that causes the moon to show the same face to the Earth at all times. One hemisphere always faces us, while the other always faces away. The lunar far side (aka the dark side) has been photographed only from spacecraft.
• The shape of the moon appears to change in a repeating cycle when viewed from the Earth because the amount of illuminated moon we see varies, depending on the moon's position in relation to the Earth and the sun. We see the full moon when the sun is directly behind us, illuminating a full hemisphere of the moon when it is directly in front of us. The new moon, when the moon is darkened, occurs when the moon is almost directly between Earth and the sun—the sun's light illuminates only the far side of the moon (the side we can't see from Earth).
• The moon orbits the Earth at an average speed of 2,300 miles an hour (3,700 kilometers an hour).
• The moon's gravitational pull on the Earth is the main cause of the rise and fall of ocean tides. The moon's gravitational pull causes two bulges of water on the Earth's oceans—one where ocean waters face the moon and the pull is strongest and one where ocean waters face away from the moon and the pull is weakest. Both bulges cause high tides. These are high tides. As the Earth rotates, the bulges move around it, one always facing the moon, the other directly opposite. The combined forces of gravity, the Earth's rotation, and other factors usually cause two high tides and two low tides each day.
Apollo 17 mission Commander Eugene Cernan checks out the lunar roving vehicle (LRV) at the Taurus-Littrow landing site in December 1972. LRVs, also called moon buggies, are electric vehicles designed to expand astronauts' range of exploration on the low-gravity surface of the moon. The east end of the moon's South Massif rises in the background at right.
• The airless lunar surface bakes in the sun at up to 243 degrees Fahrenheit (117 degrees Celsius) for two weeks at a time (the lunar day lasts about a month). Then, for an equal period, the same spot is in the dark. The dark side cools to about -272 degrees Fahrenheit (-169 degrees Celsius).
• The rocks and soil brought back by Apollo missions are extremely dry; the moon has no indigenous water. However, the moon is bombarded by water-laden comets and meteoroids. Most of this water is lost to space, but some is trapped in permanently shadowed areas near both poles of the moon.
• To the unaided eye, the bright lunar highlands and the dark maria (Latin for "seas") make up the "man in the moon." A telescope shows that they consist of a great variety of round impact features—scars left by objects that struck the moon long ago. The largest scars are the impact basins, ranging up to about 1,500 miles (2,500 kilometers) across. The basins were flooded with lava some time after the titanic collisions that formed them. The dark lava flows are what the eye discerns as maria.
• On the moon there are no mountains like the Himalaya, produced by one tectonic plate bumping into another. There is no continental drift on the moon. Everywhere, the moon is sheathed by rocky rubble created by constant bombardment by meteoroids, asteroids, and comets.
• No cheese has ever been found on the moon.