Exploring the Moon

The best place to start in Astronomy has to be our nearest neighbour in space and the only body in the solar system which we’ve visited in person, namely the Moon.

The Moon is a wonderful object for the budding astronomer to explore, even with just a pair of humble binoculars. Straight away, you’ll see so much detail that you’ll probably be left reeling for days.

The Moon emits no light of its own and so most of the light coming from the Moon is actually reflected sunlight. A small portion of the Moon’s light is actually reflected Earth-light!

The Phases of the Moon

The Moon orbits the Earth roughly once every 28 days and consequently it changes its appearance constantly, following a simple, predictable pattern which we call its phase.

At the start of the Lunar Month, we say the Moon is ‘New’. There’s not much to see at this time because the New Moon is nearly always very close to the Sun in the sky (not physically!) and only visible for a short time after sunset. A day or so later and you should see a very thin sliver for about an hour or so after the sun has gone down.

About seven days later, the Moon looks like a semi-circle. We call this phase the first quarter. Notice there’s a line which divides the Moon into two sections. One section of the Moon is illuminated by the Sun whilst the other is in shadow. We call this line the Terminator and it’s a very useful feature in that it’s very easy to see structures on the Moon’s surface around the Terminator. This is where the contrast between light and dark is at its greatest, and the shadows are strongest.

Seven days later after first quarter, the Moon shows us its full face, which we call the Full Moon. Conversely, this phase isn’t very useful for astronomers largely because the Terminator isn’t there and so we can’t see much surface detail. Also the Full Moon can be exceptionally bright, sometimes painfully bright.

Seven days later and the Moon once again presents as a semi-circle. This is the third quarter. Note that there is no second quarter! Confusing, eh?

Seven days later and we’re back to the New Moon, and the cycle repeats.

Craters

Exploring the Moon is an endlessly fascinating pursuit. Even a small telescope or a pair of binoculars will show you some of the bigger craters. All of the big craters have names, mostly those of famous scientists or important artisans. For instance, the crater Copernicus is named after the Polish astronomer, Nicolaus Copernicus, who helped us understand how the Earth moves around the Sun in space.

Copernicus is one of the biggest craters on the Moon. It lies on the Western plains in a region called Oceanus Procellarum, (Latin for Ocean of Storms). This crater was formed when a very large asteroid impacted on the Moon several million years ago, which is fairly recent, in Geological terms anyway. We know this because we can see that the rock in the middle of the crater isn’t made of solidified lava like most of the surrounding ‘Sea’. Copernicus is surrounded by rays, which spread outwards from the site of the original impact, sometimes for enormous distances. These rays were caused by debris flying out of the crater when the original asteroid landed.

The Moon is actually pretty small, only about a quarter the size of the Earth. For reasons which we’ll explore in later chapters, we always see the same side of the Moon and it’s only recently, thanks to space probes and robot landers, that we’ve been able to make detailed maps of the so-called far side of the Moon.

The Moon is, as far as we know, completely lifeless. It’s too small to hold on to any kind of atmosphere so there’s virtually no chance that any kind of life as we know it could survive. And because it has little or no atmosphere, there’s nothing to protect the Moon from incoming meteors. Hence, the Moon’s surface is very heavily pock-marked with craters from impacts which go back millions and millions of years. On Earth, we have a good thick atmosphere which gives rise to a fairly active weather system. The Moon has no tangible atmosphere and therefore no weather systems so there’s no wind and rain to erode those craters. In many ways, the Moon is something of a time capsule from another age, at time when the solar system was just forming and the early Moon and the Earth were heavily bombarded with space debris.

The Apollo Missions

We do know that the Moon was molten for a long period. We learned this from the Apollo Moon Landings at the end of the nineteen sixties and early seventies, and the dark areas on the Moon, which the ancients christened ‘Seas’ were once lava plains, covered in molten rock. That molten rock cooled over time and became comparatively smooth, which looks slightly darker. Hence, the ancients assumed that they were seas.

Is the Moon a dead world? Far from it. The Moon is still quite active geologically speaking. We know this because the astronauts from the Apollo missions left instruments behind that recorded evidence of Moon-quakes, which are just like Earth-quakes but they happen on the Moon. Recent space probes are still surveying the Moon’s surface and we can see signs that the surface features are changing, with small valleys forming. This suggests that the Moon’s crust is expanding. Also, small meteors occasionally impact on the Moon, and whilst this is rare, we do the odd flash from the Moon’s surface although they’re so brief that you need to be looking exactly the right area of the Moon at exactly the right time.

The Moon is a fascinating object to explore mainly because there’s so much to explore. Even after forty years of viewing the Moon through a variety of telescopes, I can truthfully say I’ve never been bored once.

Now, this is just the start of the journey and there is much to learn. But before we start, just go outside and see if it’s clear. Wrap up warm and stay safe.