It’s Moon week, according to us has officially declared the week 14-21 March, Moon Week. Why, because being a week starting on a Wednesday bears almost no resemblance to any other official week so it shouldn’t clash with anything else. At the moment the moon is slowly disappearing from the sky and generally not annoying deep sky imagers so it’s a good time to remember the moon and celebrate the good things it does for us, like stabilising the tilt in our axis to give us stable seasons rather than the death and destruction that we would otherwise have (maybe slightly over dramatic). So Moon Week is an excuse for to write some stuff about the Moon so we can all get to know the Mon a bit better.

The Moon is quite large and nearly everyone on the planet can’t help but be aware of its existence. Our biggest satellite is about 400,000km away, or about 1.3 light seconds. Its diameter is 3475km and has a mass of about 0.0123 of that of the Earth. If you ever saw the pictures of the astronauts that went to the Moon you might have seen how they walked with a kind of skipping jump at times. This is because the gravity on the Moon is only 0.166 of what we have on Earth.

Artist’s impression of the moment of impact (Credit: Caltech/NASA)

So how did the Moon start? This is a question that has puzzled astronomers for centuries, and indeed anyone that has spent time looking up at the night sky and seeing the big bright thing. The running theory at the moment is that a big Mars sized planet smacked into the early Earth and ejected a whole bunch of material that coalesced into what is now the Moon, quite an amazing set of events and a rather traumatic occurance for the early Earth, but as it happens, the best possible outcome for the establishment of life on our planet and the eventual existence of us. A slightly more recent theory was that when the planet sized ball hit the young Earth the combined mass was akin to a donut shape of molten rock and debris that over time reformed back into the Earth. This donut shaped mess of the Earth and the Moon was probably quite large and possibly even as big as Saturn’s rings. Inside the outer bits of the donut was the debris that would become the Moon. This donut theory allows scientists to offer an explanation of how the Earth and Moon are so similar in composition.

All of this was supposed to have happened about 30-50 million years after the formation of the Earth, about 4.5 billion years ago. Once the whole thing settles down, the Earth was spinning quite quickly and so was the Moon, but over time the Moon slowed down in it’s spin and eventually became tidely locked with the Earth which is why the same face of the Moon always is pointing towards us. Basically for one rotation around the Earth the Moon spins once, is kind of how it works. The Moon also has the effect on the Earth of slowing our spin due to the friction created by the tides. When the Earth first formed is was spinning at a rate of one rotation every few hours. The Moon was also a lot closer and as it has slowed down, so it has drifted a little further away.

The Moon from the International Space Station (Credit: NASA)

The slowing down of the Earth is estimated (by NASA) to be about 1.4 milliseconds over the last 100 years. That is a tiny amount, only about a second and a half every 100,000 years or a few hundred million years to slow down an hour – So over a few billion years it can slow down quite a lot. At the same time the Moon is getting further away from us to conserve energy in the system, at about 40mm per year – so it isn’t going to disappear anytime soon and this increase in distance is hardly noticeable within the 43000km variation in the distance anyway.