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Mark Thompson's guide to the Moon

Mark Thompson
Aug 24, 2011, 7:00 UTC

Sen—There can be few objects that have inspired both artists and scientists as much as the Moon. Perhaps surprisingly its appearance has barely changed in the thousands of years that mankind has walked the Earth and ancient civilisations enjoyed much the same view as the one we see today. During the Moon's relentless orbit around the Earth it has witnessed civilisations come and go, entire species evolve and die out and even continents slowly shift. The one thing that has changed over all those years though is our understanding of it, and its still giving us plenty of surprises.

As natural planetary satellites go, the Moon is actually quite large with a diameter of 3,476km (2,155 miles) around the equator. It orbits the Earth at an average distance of 384,400km (238,000 miles) but this varies from its closest, or perigee at 362,570km (225,000 miles) to its most distant point, or apogee of 405,410km (251,000 miles). There are a couple of things people will always think of when you mention the Moon: craters and phases which can both be observed without a telescope.

The phases of the Moon are simple to understand and anyone who has looked at it over a series of nights will notice that it changes progressively night after night with a whole cycle taking about a month. In fact the word month has its origin in the word Moon relating to the approximate length of a full lunar phase cycle. To understand the phases its important to realise that we only see the Moon because it’s a sphere and reflects sunlight - turn the Sun off and the Moon would no longer be visible.

We see the phases change as the Moon orbits around the Earth and the angle between the Sun and Moon alters. During a full Moon, the Sun and Moon are opposite each other in the sky and we see the fully illuminated or daytime face, but at new Moon they are both in the same direction and we see the night time portion of the Moon. As it moves around the Earth, the angle between the Earth, Sun and Moon changes and we see varying amounts of the daytime/nighttime side.The line between the illuminated and un-illuminated faces is called the terminator and its down this line where the Sun is just rising or setting.

From an observational point of view, the surface features are much more prominent if observed when they are near the terminator. The low altitude of the Sun from that point means the shadows cast by the features are much longer making them stand out clearly against the lunar surface. The worst time to observe the Moon is when it’s full and the shadows are minimal.

The phases of the Moon are a little more complicated than I've just explained though because the orbit of the Moon around the Earth is very slightly tilted with respect to the Earth's orbit around the Sun. If it wasn't then every time we had a full Moon the Earth would block sunlight from reaching the Moon and we would see a lunar eclipse. Clearly we don't have one every month and its because the Moon's orbit is tilted that on most occasions the Moon is slightly above or below the Earth's shadow.

Look at the Moon more closely and you will see dark grey patches, turn binoculars or even a telescope on it and some will turn into great plains while others turn into cavernous craters. The craters were created by meteoric impacts where pieces of space rock smashed into the lunar surface. We see evidence of this process throughout the Solar System even here on Earth. The larger plains, or mare as they are properly called, are the aftermath of much larger impacts that have cracked the lunar surface allowing molten lava to seep up through the mantle. The lava solidifies over time leaving the plains we see today. Before good quality telescopes it was thought these great plains were actually lunar seas.

Another effect of the Moon's orbit around the Earth are the tides. Like the Earth, the Moon has a gravitational pull and as a result it pulls on the Earth producing a bulge. As the Earth spins once on its axis it 'passes underneath' the bulge which we then experience as a tide. There are actually two bulges, one pointing roughly toward the Moon, the other in the opposite direction. When a location passes under the bulge it’s seen as high tide, hence we see two every day.

This bulge is pretty crucial and is having a big impact on the Earth-Moon system. You would think that the bulge lies directly between the Earth and Moon, given that it’s the pull of the Moon's gravity that causes it. It turns out that the rotation of the Earth drags the bulge a little ahead of the Moon in its orbit. As it lays ahead of the Moon, the extra 'lump' of material produces a little extra pull on the Moon causing it to accelerate in its orbit. If you accelerate an orbiting object, it moves into a higher orbit -in other words, it moves further away. Thanks to the Apollo astronauts who left a special mirror on the surface, we can now accurately measure its distance and have found that the Moon is moving away from the Earth at a rate of 3.8cm per year!

It’s not only the Earth that experiences the tides, the Moon too has tides, though to a much lesser degree. The gravitational pull from Earth acts to distort the Moon and produce a lunar tidal bulge toward the Earth. When the Moon first formed it was spinning much faster than it does today and its rotation displaced the tidal bulge from its alignment between the Earth and Moon. The Earth's gravitational pull still acted upon this bulge causing a braking effect on the Moon's rotation. Over many millions of years this tidal interaction caused the Moon to slow down so much that it now rotates once on its axis for every orbit around the Earth, every 29.5 days. It’s an effect called captured or synchronous rotation and its result is that we now only ever see one half of the Moon from Earth. In reality we see can see a little more than 50% but this is due to the Moon's orbital properties allowing us to glance a little further around.

With the Moon moving away from Earth it would be reasonable to assume that at some point they were in the same place. It is believed that the Moon was in fact once part of the Earth. At the time the Earth formed, the Solar System was a war zone with large chunks of rock and proto-planets flying around at ballistic speed. One piece about the size of Mars is thought to have smashed into the Earth throwing vast amounts of material into orbit. It’s believed that most of the heavy elements settled back on Earth while the lighter material stayed in orbit. Recent studies suggest that two moons could have formed, sharing the same orbit, which ultimately collided forming the Moon we see today. This new theory nicely accounts for the observation that one side of the Moon seems to have a much thicker crust which is now thought to be the remains of the Moon's ancient companion.

Perhaps one of the most incredible discoveries in recent years was the discovery of water ice in some of the deep lunar craters. In these deep craters, that remain almost permanently in shadow, temperatures remain sub zero all year round allowing the ice crystals to form. This discovery opens up tantalising possibilities for future space exploration. The water molecules on the Moon could be harnessed for and purified for future explorers to drink. Taking this a step further, separate the water molecules into their hydrogen and oxygen components and they could be used to create rocket fuel for further onward exploration. No longer can we consider the Moon as a lifeless and hostile place, instead its becoming more likely that mankind's next step out into the Solar System will involve using the Moon as an outpost for future giant leaps!