Earth’s Nearest Neighbour


Welcome to the fifth post from the Science Geek. To mark the 45th anniversary of the first manned Moon landings, for the next few posts I have decided to write about the Moon.

I hope you have enjoyed reading my previous posts and will enjoy reading this one. As always, please let me know if you have any comments.


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.

phases of the moonr

Phases of the Moon (from NASA website)

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

An Odd Coincidence

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 – oval-shaped rather than circular.  This means that the apparent size 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.


The Moon covering the Sun in a total solar eclipse (from NASA website)

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 will have noticed that it appears not to rotate. In fact it does rotate but it rotates in exactly the same amount of time it revolves around the Earth, as shown below:

moon rotation

The Moon’s Rotation

As you can see in the diagram, one side of the moon, the side marked with the arrow, always faces the Earth. The other side of the Moon, shaded dark grey, 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.

Near and far sides of the Moon

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 ( 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.

There are also many rather dubious videos on the internet which claim to be alien bases on the far side of the moon e.g.

Needless to say the Science Geek does not accept any of these claims!

Next Posts

In subsequent posts, I shall talk about future exploration of the moon. It is now 42 years since the last astronaut left the Moon.

When are we likely to go there again ?

What would we need to do to build moon bases ?

I will also talk about tidal acceleration, which is the scientific term to describe the fact that the moon is slowly moving away from the Earth and how this affects the way we keep time on Earth.

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.

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