A Brief Guide to the Moon

BASIC MOON DATA

Diameter: 3,475.5 km
Mean Distance from Centre of the Earth: 384,000 km
Rotates on its Axis in: 27.3 days
Phases repeat every: 29.5 days
Speed around the Earth: 3,680 km/h

The Moon is the Earth's only major natural satellite. There are possibly some tiny rocks (just metres across) which go around us in the same orbit as the Moon, but to all intents and purposes we only have one Moon. You can also ignore misinformation from programmes and books that say we have more than one - they are wrong (see my article on Cruithne).

Just by glancing up at the Moon you cannot fail to see it's made up of light and dark patches. Those olden astronomers took the darker areas to be seas against the brighter land.

Even though we know this not to be the case, the names of the 'seas' and sea-like features have remained in use as you can see here:

The Moon's Story

The Moon was probably made with the Earth about 4.5 billion years ago. How it was made is an interesting question, with shifting answers. The most popular theory involves a large object smashing into the Earth which blasted some mantle material into space forming a ring around our planet. Over a relatively short time, maybe just one year, this material joined together to form the Moon. I still think the Moon People made it out of Green Cheese (the Lunar-Cheddar Theory), but I believe I'm in the minority.

As for the reason why the Moon has craters we have to look at the early solar system. During this time lots of objects were flying here, there and everywhere and smashing into everything that got in the way. The Earth received its fair share, but due to our atmosphere and plate tectonics virtually all of our early craters have been wiped clean from the surface. Not so on the Moon. It has no atmosphere, it's too small to hold on to any significant one, so everything was just left in perfect condition, craters and all.

The 'seas' (Maria) were formed during the heaviest cratering sessions when the surface was cracked open causing the molten material from below to leak out to form these vast dark lava flow landscapes.

So, what about the far side? The Russian Luna 3 took the first pictures of the other side back in 1959 and found that it too is covered in craters, but there are no real 'seas'.

The Moving Moon

The Moon takes 27.3 days (the Sidereal Month) to orbit the Earth, and rotates on its axis in exactly the same time. This means the Moon always shows us the same face. Well, this isn't strictly true because there is an amount of wobbling due to various tilts and the non-circular orbit which actually allows us to see 59% of the surface - an effect called Libration.

The phases of the Moon over the month (29.5 days)

Now you may think that, from the Sidereal month mentioned above, the phases repeat themselves over 27.3 days. No, no no. Because the Earth is also travelling in orbit around the Sun at the same time, a complete set of Moon phases (i.e. Full Moon to Full Moon) takes 29.5 days (called the Synodic Month). This length of about one month is indeed where the word 'month' comes from, it originally being 'Mooneth'.

Not only that, but the Moon's orbit around us is not a perfect circle - it's slightly squashed, like an oval. Boffins call this shape an elliptical orbit. This has the effect of making the Moon look a bit bigger and then a bit smaller over the month as it moves from close in (called perigee) to further away (apogee). The next diagram that follows shows what is going on.

The Moon's path around the Earth

This change in distance affects eclipses, as when the Moon is closer it can completely cover the Sun forming a Total Solar Eclipse. Further away, nearer apogee, we see an Annular Eclipse.

Moon Watching

My map below will help you identify the light and dark areas of the Moon that you can see simply by gazing up into the sky. There's not much else you can do without binoculars or a telescope, although some people claim you can occasionally see craters with the unaided eye. Presumably this is along the 'terminator' described below.

Now we've found a pair of dusty binoculars and we've moved up a gear. One interesting place to look is the boundary between sunlight and darkness, the Moon's day and night, called the terminator. Due to the Sun catching the features either side of this line, you'll see mountains, craters with or without central peaks, faults, rilles, domes, ridges and valleys, all being highlighted and casting shadows, and all this is changing hour by hour due to the Sun's illumination as the Moon travels around the Earth.

Anton's Official Map of the Moon

Full Moon is not the best time to study though, the lack of shadows make the majority of the craters 'invisible', but it is the best time to observe all the 'seas'.

Learning the Moon is the same process as learning the constellations. If you take it slowly, you'll soon be able to work your way around with ease.

© Anton Vamplew 2008