As our closest neighbor, the moon is one of the most easily observed objects in the sky; in fact, only the Sun is brighter. And since prehistoric times, the moon has been a source of fascination and mystery.

It was the ancient Greek astronomer Parmenides (512-400 B.C.) who made one of the earliest discoveries about the moon: that moonlight is reflected sunlight. Since then, we’ve unraveled many of the moon’s mysteries, and have gained insight into the relationship between the moon and Earth. We’ve also made it a primary focus of space exploration. The moon is the only celestial body visited by humans, and is the most explored object in the universe. Since 1958, more than 70 spacecraft have been sent to the moon and 12 astronauts have landed on its surface.

The moon is approximately 4.6 billion years old, about the same age as Earth. Theories abound as to the moon’s origin--one of the most widely accepted is the collision theory. According to this theory, when the Earth was young, it was struck by an asteroid or comet, which sent a cloud of matter into space. This matter, it is thought, eventually formed the moon. Still another theory, the simultaneous creation theory, states that both bodies formed at the same time, out of material floating through space. Because the Earth and the moon have substantially different compositions, however, this theory is not as commonly accepted. There is also the belief that the moon may have formed elsewhere in the solar system, and was captured by Earth as it traveled by the planet. This theory also has a flaw, however, because the bodies are relatively similar in size, and capture is more probable if one of the objects is significantly larger.

Stretching about 2,160 miles across, and located about 238,900 miles away from Earth, the moon features a rocky surface dotted with boulders and scarred by approximately half a million craters. The largest of these craters, the Imbrium Basin in the Sea of Rains, is 700 miles wide and is the true identity of the so-called “Man in the Moon.” The moon is covered by a layer of soil ranging from 5 feet to 20 feet deep and made of pieces of rock and glass. Unlike Earth, the moon appears to consist of very little metal. Lunar rocks have been found to contain a wide range of elements, including calcium, magnesium, iron, aluminum, phosphorous, potassium and titanium. Lunar rocks come in two varieties: breccia, made of pieces of rock and soil, and basalt, made of hardened lava. Astronauts have brought back approximately 842 pounds of lunar rock and soil.