The Moon – Our Nearest Neighbour

Updated April 2026

Our nearest neighbour in space, the Moon is by far the largest and the most noticeable object in the night sky. It has fascinated humans since the beginning of history, because it is close enough for surface features to be clearly seen with the naked eye. Up until the dawn of the scientific age, many thinkers thought there might be life on the moon. Indeed, the darker areas of the moon are called ‘maria’ – from the Latin for sea – because early astronomers thought the moon was partially covered in seas and oceans.

Phases of the moon

To anyone who has observed the moon for a while, the phases of the moon are noticeable. Starting with with a new moon, when the Moon is not visible, over a 29.5 day cycle the sunlit part of the Moon gets larger or waxes through to a crescent phase to a half moon (known as the first quarter) to a full moon, when the whole moon is illuminated.  It then gets smaller or wanes  back to a half moon (known as the last quarter), then to a crescent and then finally to a new moon.

This 29.5 day cycle was used in many calendars throughout history and forms the basis of the month.

The apparent sizes of the Sun and the Moon

The Moon is roughly 400 times smaller in diameter than the Sun, but by coincidence it is also approximately 400 times closer to the Earth than the Sun. This means that, when viewed from Earth, the Sun and the Moon appear to be almost exactly the same size.

In fact it is not quite as simple as that, because the Earth’s orbit around the Sun and the Moon’s orbit around the Earth are both elliptical the apparent sizes of the Sun and the Moon vary and appear to be larger when they are nearer to the Earth. Therefore, at certain times the Moon will appear to be slightly larger than the Sun and at other times it will appear to be slightly smaller. A total eclipse, when the Moon completely obscures the Sun  can only occur when the Moon appears to be larger than the Sun.

 

When the Moon appears to be  smaller than the Sun, a total eclipse is not possible. In fact the Moon is slowly getting further away from the Earth so in the far future there will be no total eclipses visible on Earth. I will say more about this in a future post.

The Near and Far Sides of the Moon

Anyone who has observed the Moon over successive days will have noticed that, although it waxes and wanes, it appears not to rotate. In fact, the Moon does rotate but completes one rotation on its axis in the same period of time it completes one orbit around the Earth.:

As you can see in the diagram, one side of the Moon, the side marked with the yellow dot, always faces the Earth. The other side of the Moon, centred on the black dot, is known as the ‘far side’ and always faces away.  

Throughout history the far side of the moon remained an enigma because it is completely invisible from Earth and for this reason it is sometimes called the “dark side of the moon”, even though it gets exactly the same amount of sunlight as the near side.It was only in 1959 that the first images of the far side were taken by the Luna 3 Soviet space probe. These caused immense excitement around the world. These and subsequent pictures showed that far side looks very different from the near side. It has a battered, heavily cratered appearance with few maria.

The Near and Far sides of the Moon (Image from NASA)

The Far Side of the Moon and Conspiracy Theories

Because the far side of the moon is completely hidden from any Earth-based telescope, it has provided a fertile breeding ground for conspiracy theorists. Indeed, the UFO casebook website (http://www.ufocasebook.com/moon.html) claims not only that there is a huge alien base on the far side of the moon but also that it was spotted by the Apollo astronauts who were then told by the NASA management to keep quiet about it.

And Finally…

I hope you have enjoyed this post. It is part of series of a series of posts on the Moon, other related posts include:

• Humans living and working on the Moon
• The Apollo missions which landed humans on the Moon in the 1960s and 1970s
• How Latitude and longitude are defined on the Moon
• How Libration lets us see some of Moon’s far side
• How the Moon causes tides
• Are lunar stationary orbits possible? (the lunar equivalent of geostationary orbits)

There is also an e-book available The Short Guide to the Moon