Earth's Moon Facts
Telescopic view of the moon as seen from Earth, the dark areas are lower in altitude.
  • It is thought that the Moon formed 4.5 billion years ago from vaporized rock after the Earth collided with a planet size object.
  • The Moon is around a quarter the size of Earth with a diameter of 2158 miles (3474 km).
  • The Moon orbits the Earth at an average distance of 239,000 miles (384,000 km).
  • The Moon spins on its own axis every 27.3 days, it also takes the same time to complete one orbit of Earth, as a result we only ever see one side of its surface.
  • The Moon is moving 3.8 cm further away from the Earth every year.
  • Gravity on the Moon is one sixth of that on Earth.
  • Despite the Moon's weak gravity it is strong enough to produce tides on Earth.
  • The largest crater on the Moon is 1,550 miles (2,500 km) in diameter.
  • Ice water from comets that hit the Moon has been detected at both its poles and may be present in other areas.
  • Neil Armstrong became the first man to step on the Moon on July 20, 1969.

surface of the moon
Image of the lunar surface, a landing module can be seen just below center
Moon's Surface
The Moon's surface is covered with powdery soil and rocks blasted out by meteor impacts. The light areas we see on the Moon are called "Highlands", the dark areas are called "Maria" which are lower in altitude.
Moon's Atmosphere
The Moon has no real atmosphere instead it is surrounded by an extremely thin layer of gases brought by solar winds, mostly hydrogen and helium. It is sometimes referred to as an exosphere.
Moon's Temperature
The temperature on the Moon ranges from extremely hot to extremely cold. During the day temperatures can reach over 100C (212F), at night they can fall below -150C (-238F).

Life on the Moon
It is highly unlikely that the Moon could support life of any kind.
Origin of Name
The name derives from the Germanic word moon which is related to the Latin mensis meaning month.
Lunar derives from the Roman name for the Moon, Luna.

The Moon's Effect on Earth

aurora at jupiter's pole
The gravitational effects of the moon produces tides on Earth
The moon is incredibly important to Earth, without it our stable environment would simply not exist, and the abundance and variety of life would not be present on our planet. Our moon is unusual in the solar system in terms of its size in comparison with its host planet, the moon is a quarter the size of the Earth in radius, other moons in the solar system are tiny when compared to the planets they orbit. In fact some astronomers believe that the Earth and the moon are a double planet, with each essentially orbiting around the other.

Due to its size and close proximity the moon has an enormous effect on the Earth. As is commonly known the moon produces ocean and sea tides, this is because as it orbits the Earth its gravity forces the water towards it, creating bulges which are forced along as the moon continues its orbit. These gravitational effects produce the high and low tides which are most commonly produced twice a day on Earth. The moon’s gravity actually produces a similar effect on the land of the Earth but it is much less noticeable.

But the moon does more than produce tides, its presence provides our planet with a stable rotation. The reason we have seasons is because Earth rotates on its axis at an angle of 23 degrees, this means that at different times of the year the hemispheres are either facing towards the sun, so receive more sunlight, or facing away resulting in less sunlight. Without the stability the moon provides the angle at which the Earth revolves would frequently change, rotating at various angles including on its side and even upside down! This would produce an incredibly unstable environment, resulting in constant severe flooding and extreme local temperature variations, under these conditions it is possible that evolution would not have been able to take hold and even simple organisms would not be able to survive. So all in all we have lot to thank our nearest neighbor for.

How Did The Moon Form?

earth moon collision
Bodies colliding in the early solar system
How did Earth end up with such a large moon and 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 a large 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 during its formation the moon was covered in a deep molten lava ocean, they were also quite surprised to discover that the rock and soil samples 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. 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 ocean 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.