Image taken by an astronaut on the lunar surface,
the landing module can be seen just below center

Origin of Name

Life on the Moon

Formation of the Moon

The moon is in no way the biggest in the solar system but it is the largest in relation to the size of the planet it orbits, around a quarter the size of Earth. Some astronomers even refer to the Earth and the moon as a double planet. So how did Earth end up with such a large satellite, how did it form? Over the last 150 years many astronomers have attempted to answer this question.

The first theory was put forward by the French astronomer Edouard Roche in 1873. He believed that the Earth and the moon formed side by side at the same time and from the same materials. In order for this theory to work the Earth and the moon would need to have the same composition, but they don’t. The moon is far less dense than the Earth with a much lower iron content, unlike Earth it doesn’t have an iron core.

In 1878 George Darwin, son of Charles Darwin, believed that the moon was in fact moving away from Earth, a theory that was proved correct almost 100 years later, at the rate of almost 4 centimeters every year. He put forward the idea that if you imagine this theory in reverse, going backwards in time, the moon would get closer to Earth every year until eventually it would collide and join together. Darwin concluded that part of the Earth must have separated when the planet was in a molten state, forming the moon. The mathematics of this idea simply didn’t add up and the theory was discounted.

In 1909 the American astronomer Thomas Jefferson Jackson See came up with the “capture theory”. This involved the moon forming in another part of the solar system which at some point came to close to the Earth and was captured by its gravity. This theory explained the differences in composition between the Earth and the moon, if it formed elsewhere in the solar system its composition could be very different. The large hole in this idea is that the moon is thought to be far too large to have been captured by Earth’s gravity.

When the Apollo 11 astronauts returned from the first manned lunar landing in 1969 they brought with them soil samples and rocks from the moon. Geologists analyzing the samples concluded that after its formation the moon’s surface was covered with molten lava. They also found another big surprise, the rock and soil samples from the moon were similar to what we find on Earth’s surface. In the 1970’s the American astronomer William Hartmann used these findings to create a new hypothesis called “the giant impact theory”. He proposed that another planet around the same size as Mars formed near our planet and at some point collided with Earth. This collision was so massive it kick started our planet’s 24 hour rotation. The collision launched a huge amount of debris around the Earth which formed into two clumps, one composed mainly of the smaller planet’s iron core, the other primarily made of crustal material from both planets. The iron clump re-collided with Earth and was absorbed into the planet, the remaining clump went on to form the moon. This explained the lack of iron in the composition of the moon and that the tremendous heat generated by the collision would have indeed created a molten lunar surface in its early formation.

Although this theory was initially ignored it eventually was accepted by leading astronomers around the world as the most acceptable explanation we have for the formation of the moon.

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Images and Video

The Moon eclipsing the Sun

Apollo 11 Moon Landing

A "Harvest Moon"

Future and Past Mission to the Moon