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Science Magazine Feature

The Countryside Guide to… The Moon

Since the dawn of humanity, the Moon has been our constant companion. As well as lighting our way, controlling our tides and influencing our seasons, it’s a source of endless fascination. Whether you’re looking at it from a mystical or scientific viewpoint, our nearest neighbour has a lot of secrets to give up.

From seasoned star spotters to astronomical amateurs, the Moon is one heavenly body everyone can identify. It’s found floating in space just under 250,000 miles from Earth, which is a mere hop, skip and jump in galactic terms when you consider that Venus – our next-door planet – can be anywhere from 25 million to 160 million miles away. Though it doesn’t glow of its own accord, the Moon’s proximity to Earth makes it appear super-bright as its surface reflects the light of the Sun.

So far, over 100 robotic crafts have landed on the Moon and 12 astronauts have set foot on its surface. Although no one has walked on the Moon since NASA’s Apollo 17 mission in 1972, many nations continue explorations. Why? To build scientific knowledge, investigate the possibility of lunar settlements and test the Moon as a base for flights to Mars and into deeper space.

It’s certainly not made of cheese
At about a quarter of the size of Earth, the Moon comprises a partly molten core, most likely made from iron. Around this is the mantle – a thick, mineral-dense layer that makes up most of the Moon’s bulk. The whole thing is then encased in a crust, on average 30 miles deep and made mainly of an igneous rock called anorthosite, rich in feldspar.

Because of gravitational spin, the Moon is broadly a ball, but scientific observations have revealed it to be more specifically egg-shaped. This distortion may have been caused by the pull of Earth’s gravity. The large end of the egg faces Earth and the pointy end faces away. Because the Moon has only a minor tilt on its axis, some polar areas are never exposed to the Sun and here, in shadowy craters, water ice can be found.

Collecting samples of lunar soil has been a major preoccupation of the various missions to the Moon. Because of the extensive analysis of these samples, we know that lunar ‘regolith’, as it’s called, is made up of dust, gravel and broken rocks consisting of silicon, calcium, oxygen, aluminium, iron, magnesium and titanium. At its finest, the regolith is like a sticky dust of ground glass, which astronauts returning from moonwalks found clinging to their spacesuits and coating their landing craft.

The creation event
Exactly how the Moon came to be is a mystery – no one was there to witness its birth, after all. Over the years, there’s been lots of speculation about the likely sequence of events. Was the Moon a rogue body floating through space that was sucked in by Earth’s gravity? Is it a small twin, created at the same time as Earth and locked in sibling orbit with it? Or is it made from some bits that flew off Earth as it spun at ultra-high velocity through space?

Today, much credit is given to the possibility that the Moon is the result of a ‘giant impact’ event, where Earth and a Mars-sized planet collided. The resulting debris, caught in orbit, gathered together and slowly transformed into the Moon as we know it today.

Near side and far side
Over 27 days, the Moon completes a full orbit of Earth – a fact reflected in our word ‘month’, derived from ‘moon’. Coincidentally, it also takes 27 days for the Moon to complete a full rotation on its axis. Because of this ‘synchronous rotation’, wherever you are on Earth, you’ll only ever see one side of the Moon. Logically, the part that faces us is known as the ‘near side’, while the other is known as the ‘far side’ or ‘dark side’ – though technically it’s not dark, just unseen.

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