THE MOON The moon is Earth's only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles.

The same side of the moon always faces the Earth. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA's Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969.

If you were standing on the moon, the sky would always appear dark, even during the daytime. Also, from any spot on the moon (except on the far side of the moon where you cannot see the Earth), the Earth would always be in the same place in the sky; the phase of the Earth changes and the Earth rotates, displaying various continents.

THE MOON'S ORBIT
The moon is about 384,000 km from Earth on average. At its closest approach (the lunar perigee) the moon is  356,410 km from the Earth. At its farthest approach (its apogee) the moon is 406,700 km from the Earth.

The moon revolves around the Earth in about 27 days 8 hours. It rotates around its own axis in the same amount of time. The same side of the moon always faces the Earth; it is in a synchronous rotation with the Earth.



SAROS
The saros is the roughly 18-year periodic cycle of the Earth-Moon-Sun system. Every 6,585 days, the Earth, Moon and Sun are in exactly the same position. When there is a lunar eclipse, there will also be one exactly 6,585 days later.

SIZE
The moon's diameter is 3,476 km, 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter).

The gravitational tidal influence of the Moon on the Earth is about twice as strong as the Sun's gravitational tidal influence.

MASS AND GRAVITY
The moon's mass is about 1/81 of the Earth's mass.

The moon's gravitational force is only about 1/6 of the Earth's gravity. For example, a 42 kg person would weigh only 7 kg on the Moon.



TEMPERATURE
The temperature on the Moon ranges from daytime highs of about 130°C  to nighttime lows of about -110°C

ATMOSPHERE
The moon has no atmosphere. On the moon, the sky is always appears dark, even on the bright side (because there is no atmosphere). Also, since sound waves travel through air, the moon is silent; there can be no sound transmission on the moon.

MARE
Mare (plural maria) means "sea," but maria on the moon are plains on the moon. They are called maria because very early astronomers thought that these areas on the moon were great seas. The first moon landing was in the Mare Tranquillitatis (the Sea of Tranquility). Maria are concentrated on the side of the moon that faces the Earth; the far side has very few of these plains. Scientists don't know why this is so.

CRATERS
The surface of the moon is scarred by millions of (mostly circular) impact craters, caused by asteroids, comets, and meteorites. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in our atmosphere). Also, there is no erosion (wind or precipitation) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it.


LUNAR EXPLORATION

Astronaut Buzz Aldrin's footprint on the moon's Sea of Tranquility, from the Apollo 11 mission in 1969.

There have been many missions to the moon, including orbiters missions and moon landings. NASA's Apollo missions sent people to the moon for the first time. Apollo 11's LEM (Lunar Excursion Module) landed on the moon on July 20, 1969 with Neil Armstrong and Edwin "Buzz" Aldrin (Michael Collins was in the orbiter). Neil Armstrong was the first person to set foot on the moon. His first words upon stepping down the Lunar Module's ladder onto the lunar surface were, "That's one small step for man, one giant leap for mankind." Aldrin described the lunar scenery as "magnificent desolation." Apollo 12-17 continued lunar exploration.

MOON ROCKS
NASA astronauts have retrieved 382 kg of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks.


BLUE MOON

When two full moons occur in a single month, the second full moon is called a "Blue Moon." Another definition of the blue moon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)