Earth's Moon

Richard Bloch
11 January 2013

Above: The moon, seen from Earth on February 20, 2008 (Thom Rains)

Fast Fact: The computers on the spacecraft that allowed humans to reach the moon were many times less powerful than a smartphone!The first satellite to orbit Earth was launched 4.5 billion years ago! Of course, when talking about the moon, ”satellite” means “natural satellite,” and “launched” means “Earth's face was blown apart and thrown into space.” Humans have long observed the moon and, in more recent decades, some have actually visited it.

Aside from the sun, the moon is the only object in the sky that appears to be more than just a point of light when observed with the naked eye. In fact, so much detail is visible on the moon that ancient astronomers named its dark patches—called “maria,” which is Latin for “seas”—and many of its larger craters.

Lunar nearside with major maria and craters labelled. ("mare" is the singular of maria). Click to enlarge (Peter Freiman et al.)

Where did Earth’s moon come from? Four and a half billion years ago, when the solar system was being formed, a planet at least the size of Mars (and probably much heavier than Mars) crashed into Earth. This collision was so powerful that, within only a couple of hours, Earth's crust was blown into orbit, along with some material from the other planet. These molten fragments began to stick together, eventually forming a sphere and cooling into the moon.

Fast Fact: It would take more than 150 days to reach the moon by car travelling at 100 km/h (62 mph)The moon's outer crust soon formed solid rock. But every now and then a giant asteroid would break through the lunar surface. Molten rock would rush out from within the moon and cover part of the surface, before cooling and solidifying. The new material had a different composition than the lunar crust, and when it cooled it left dark patches on the surface. The molten rock also filled in craters, which is why there are very few craters visible in the lunar maria. Over millions of years, the interior of the moon cooled as well, so even if more collisions with big asteroids were to occur, there is no longer any molten material to spill out.

Aside from Earth, the moon is the only celestial body that humans have visited. Between 1968 and 1972, the Apollo program sent 24 different men to the moon, three of whom participated in two different missions and 12 of whom actually walked on the lunar surface. Neil Armstrong and Buzz Aldrin—the first men to set foot on the moon, on July 20, 1969—are household names. But few people remember Harrison Schmitt and Eugene Cernan, the Apollo 17 astronauts who were last human beings to walk on the moon, on December 14, 1972.

In total, the six Apollo missions that landed on the moon brought back a total of around 800 pounds of lunar material–one of the most valuable and well-protected rock collections in the world! These samples provide strong evidence for the theory that the moon originally formed part of Earth.

The moon remains one of the most closely studied objects in the solar system and a popular topic for space enthusiasts who are eager to see humans begin exploring the solar system once again!

Learn More!

Apollo 11: One Small Step (NASA) The Origin of the Moon (Gregg Herres and William K. Hartmann, Planetary Science Institute) NASA's Mighty Saturn V Moon Rocket Explained (

Richard Bloch

Richard Bloch is a student of political science and astrophysics at York University in Toronto. He spends his days with work, classes, video games, and slacking off, but spends every night either reading about astronomy or practicing it out of the city. He sleeps when he's lucky, and doesn't when he's luckier.

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