The Moon

How big is the Moon?

The Moon is a little over a quarter the size of the Earth, with a circumference of 10,917 kilometres around the equator and a radius (the distance from the core of the Moon to the surface) of just 1,737 kilometres. In relation to Earth, the Moon is much larger than would be expected and this is thought to be due to how the Moon formed.

How cold is the Moon?

There is almost no atmosphere on the Moon, which means it cannot trap heat or insulate the surface.

In full sunshine, temperatures on the Moon reach 127°C, way above boiling point. There are 13 and a half days of high temperatures followed by 13 and a half days of darkness, and once the Sun goes down the temperature at the bottom of craters can plummet to -173°C.

Is there water on the Moon?

Due to its tilt, some parts of the Moon's surface never see sunlight, allowing water ice to survive in some of its craters. When India's Chandrayaan-1 lunar orbiter passed over the north pole of the Moon in 2009 it found more than 40 craters thought to contain water ice. This confirmed a finding from the previous year that found water ice on the southern pole. In addition, the Moon contains some water trapped in its rocks.

How far from Earth is the Moon?

The Moon is orbiting Earth at an average distance of 384,400 kilometres, meaning it would take over 17 days non-stop to fly there on a commercial plane.

Its orbit is not perfectly circular, but varies between 357,000 and 407,000 kilometres away. We tend to think it is closer than it actually is simply because it is the largest celestial object in the sky. 

Is the Moon getting closer?

No. The Moon is actually gradually getting further away - every single year the Moon moves about four centimetres further out. This is because there is a small amount of friction between Earth and the tides, slowing our planet's rotation. As Earth's spin slows, the Moon is creeping away.

How long is a day on the Moon?

It takes 27 days for the Moon to go around Earth and 27 days to rotate once on its axis. Because the Moon is orbiting Earth at the same rate at which it rotates itself, this means that the same side of the Moon always faces the Earth. This is known as synchronous rotation.

Synchronous rotation is why some people call the side that never faces the planet the 'dark side' of the Moon. This is somewhat misleading, however: it is more correctly referred to as the far side of the Moon. As the Moon orbits the Earth most of its surface is bathed in sunlight at some point. 

Is there an atmosphere on the Moon?

For a long time scientists thought that there was no atmosphere on the Moon, but recent studies have confirmed that there is one.

The very thin atmosphere, known as an exosphere, contains helium, argon, neon, ammonia, methane and carbon dioxide. It also contains sodium and potassium, which are not usually found as gases in the atmospheres of Earth, Venus or Mars.

Where this atmosphere comes from is still not known. Some theories suggest that the solar winds and high energy particles are stripping material from the surface of the Moon, while others propose that evaporation of surface material might be involved or even meteor impacts. It could also be a combination of all of these effects.

Why are there so many impact craters on the Moon?

The thin atmosphere leaves very little to protect the Moon from asteroids. Early in the solar system's formation, all planets and moons were bombarded with rocks.

The thin atmosphere on the Moon has meant that the impact craters have remained prominent - because the Moon has no weather there is effectively no erosion on the celestial body. 

What is the Moon made of?

Just like Earth, the Moon can be divided into the crust, mantle and core. At its very centre, the Moon has a solid iron core with a temperature of between 1,327°C and 1427°C. This is hot enough to create a surrounding molten liquid iron outer core, but not hot enough to warm the surface.

The mantle which envelops the core is roughly 1,000 kilometres thick. During the early history of the Moon, this layer was once liquid magma and the source of the intense volcanic activity that led to the formation of the lava plains on its surface. As the magma cooled down, this process stopped.

All of this is encased in a crust, made up largely of a rock called anorthosite, which is rich in oxygen, silicon, calcium, and aluminium.

The surface is coated with lunar regolith - a fine mix of dust, broken rock and material.

While Earth's regolith is formed by erosion and weather, on the Moon it all comes from meteor impacts as the surface is blasted into fine pieces. In some places, this lunar regolith is just three metres deep, while in others parts it has settled into drifts some 20 metres deep.

How does the Moon cause the tides?

The gravitational pull of the Moon causes the water on the nearest side of Earth to bulge outwards, resulting in a high tide. Curiously, it also causes the water on the other side of the Earth to bulge outwards.

This is because the Moon's gravity is not the only force acting on the planet's water, as Earth's own gravity also has to be taken into account. The resulting tidal force is stretching and squashing Earth, resulting in the water bulging on the two opposite sides of the planet.

This is why we experience two high tides and two low tides per day.

How many times have people landed on the Moon?

All six human Moon landings to date have been part of NASA's Apollo programme, which ran between 1961 and 1972.

After the momentous achievement of Apollo 11, landing Neil Armstrong and Buzz Aldrin on the Moon on 20 July 1969, the USA landed five more missions up to 1972. Twelve people in total have walked on the surface of the Moon. During these missions, the astronauts collected samples, conducted science experiments, explored the landscape in a buggy and even played golf.

Since then, no one has been back to the Moon, however, NASA plans to place humans on the lunar surface again with the launch of Artemis III in 2025.