Moon Facts

The Ocean tides are caused mostly by the Moon (the Sun has a smaller effect):

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.

The moon's gravitational pull on the Earth 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.

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, smaller neap 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 perigee 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.

The Moon is a planet?

Our Moon is bigger than Pluto. And at roughly one-fourth the diameter of Earth, some scientists think the Moon is more like a planet. They refer to the Earth-Moon system as a "double planet." Pluto and its moon Charon are also called a double-planet system by some.

Punching bag

The Moon's heavily cratered surface is the result of intense pummeling by space rocks between 4.1 billion and 3.8 billion years ago. The scars are seen as craters and haven’t eroded much as 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 very little atmosphere there is no wind or rain, so very little surface erosion occurs.

Locked in orbit

The Moon always shows us the same face. Since both the Earth and Moon are rotating and orbiting, how can this be? 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.

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.

Is the Moon is moving away from the earth?

Every year the Earth's rotational energy propels the Moon about 3.8 centimeters higher in its orbit. Researchers say that when it formed, the Moon was about 22,500 kilometers from Earth. Now it’s approximately 450,000 kilometers away.

The rotation of the moon

The time it takes to spin once around on its own axis—is 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 phase cycle

The shape of the moon appears to change in a repeating cycle. As viewed from the Earth, 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).

• 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.

• The moon orbits the Earth at an average speed of 2,300 miles an hour (3,700 kilometers an hour).

• 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.