Our moon might only be the fifth largest moon in the Solar System, but it is unique.
It has a diameter of 3,475 kilometres, compared with the Earth害羞草研究所檚 12,756 km, so the moon has a diameter that is 27 per cent of the diameter of the planet it orbits. The four larger moons are Saturn害羞草研究所檚 moon Titan, with a diameter of 5,550 km, and Jupiter害羞草研究所檚 moons Ganymede (5,262 km), Callisto (4,820 km) and Io (3,643 km). However, Titan害羞草研究所檚 diameter is only about 5 per cent of Saturn害羞草研究所檚 120,540 km diameter, and Ganymede less than 4 per cent of Jupiter害羞草研究所檚 142,980 km diameter. This has led to the Earth-moon system being often referred to as a 害羞草研究所渄ouble planet.害羞草研究所� How did this situation come about?
The Solar System was born from the collapse of a huge cloud of cosmic gas and dust. It formed a disc, with the centre collapsing to form the sun. In the rest of the disc, several 害羞草研究所渕ini-discs害羞草研究所� formed, with their centres condensing to form the planets. In their discs, some 害羞草研究所渕ini-mini-discs害羞草研究所� condensed to form their moons. That idea works well for all the planets 害羞草研究所� except the Earth. That process would have left us with maybe a tiny moon or two, probably none at all for a small planet like ours. At the moment we still don铆t know for sure, but a widely-held theory is that there was a collision.
During the early days of the Solar System there were assorted objects moving in random orbits, resulting in collisions. This was an important part of how the planets grew. Generally, by the time the growing planets had achieved some size, most of the badly-behaved big objects had been swept up and had been already incorporated 害羞草研究所� except for at least one. It has even been named: Theia, after the Greek goddess of sight, and mother of Selene (the moon), Helios (the sun) and Eos (the dawn).
The Earth is believed to have achieved something like planet status 4.54 billion years ago. Then, 4.51 billion years ago Theia, a body about the size of Mars (around 6,500 km diameter) smashed into it at a speed of many kilometres a second. Parts of Theia and Earth were blasted off into space, where they formed a belt which gradually combined together to form the moon. This could explain why the moon seems to be a pastiche of assorted bits.
Tidal forces long ago locked the rotation of the moon so that one side faces us all the time. So until 1959, when the Soviet spacecraft Luna 3 shot behind the moon and took pictures, we had no idea what the moon害羞草研究所檚 far side looked like. We expected it to be much like the side we see. It wasn害羞草研究所檛. Our side shows huge lava flows covering much of the surface; the other side is mainly mountains and craters. Thanks to the Apollo space missions and assorted lunar orbiters, we know that the moon害羞草研究所檚 crust is much thinner on our side than it is on the far side. That may explain the huge flows of lava out onto the surface facing us. This is thought to be due to the moon having formed from material from two different bodies.
If you have never looked at the moon through binoculars, or better still, a small telescope, then you really should try it. It害羞草研究所檚 best to observe when the moon is not full, so the sun is lighting part of the lunar surface at a low angle. Those huge, dark lava flows stand out, along with countless craters of all sizes. If you look carefully, you will see the mountains are mostly walls of old, huge craters, not like mountain ranges here on Earth, produced by plate collisions. As far as we know the moon has never had any plate tectonics. It害羞草研究所檚 best to observe the moon when it is high in the sky, but if you have a clear horizon, get out the telescope and watch the moon rise. Seeing the mountains, lava flows and craters coming up behind the trees is a fascinating and beautiful sight.
Venus lies low in the sunset glow. Jupiter rises about midnight, Mars and Saturn, close together, around 4 a.m. The moon will be full on March 30.
Ken Tapping is an astronomer with the NRC害羞草研究所檚 Dominion Radio Astrophysical Observatory, Penticton.
