A casual question from a colleague recently - "Does the moon spin on its axis?" - led to this article. I answered that I thought it spins slowly, but I wasn't sure, so I decided to look more closely at our nearest astronomical neighbour.
The moon orbits the Earth at a mean distance of 384,400 kilometres. The moon's diameter is 3,476 kilometres and it has a mass of 0.073x1024 kilos. The Earth's diameter is 12756 kilometres and it has a mass of 5.90 x 1024 kilos. No other planet in our solar system has a satellite as large in comparison to the size of the planet.
The moon is the brightest object in the sky after the sun. It shines by reflecting light from the sun. The moon spins on its axis just as the Earth does, but much slower. It rotates fully on its axis in the same time it takes to revolve around the Earth, 27 days, seven hours and 43 minutes.
We always see the same side of the moon facing us - the "bright side". Because we never see the opposite side, we call it the "dark side". However, because the moon spins on its axis, all parts at the same latitude receive equal amounts of sunlight over the long lunar day (apart from permanently shaded polar regions).
As the moon orbits the Earth once a month, the angle between the Earth, sun and moon changes and we see this as the cycle of the moon's phases. The time between two successive full moons is 29.5 days, longer than the moon's orbital period (measured against the stars), since the Earth moves a significant distance along its orbit around the sun in that time.
The moon is the only extraterrestrial body to have been visited by humans. The first landing was on July 20th, 1969. I remember watching it on TV in my "digs" in Ranelagh - do you remember where you were? The last lunar landing was in December 1972. And these lunar landings were not hoaxes, as I explained in my column of May 2nd!
The gravitational pull of the moon on the Earth is mainly responsible for the tides. The moon's pull is stronger on the side of the Earth facing the moon and weaker on the opposite side. The Earth is not perfectly rigid, and it becomes slightly stretched out along a line towards the moon, causing two small bulges, one in the direction of the moon, one directly opposite. The bulge effects are more pronounced in the water than in the solid crust of the Earth. The Earth rotates much faster than the moon moves in its orbit and so the bulges move around the Earth once a day, giving two high tides per day.
The rotation of the Earth on its axis once a day carries the bulge slightly ahead of the point directly beneath the moon. Consequently the force between the Earth and moon is not precisely along the line between their centres. The net result is a force that slows the Earth's rotation by about 1.5 milliseconds a century and raises the moon into a higher orbit by about 3.8 centimetres every year.
The moon has no atmosphere. Any early atmosphere it might have had escaped from the moon's feeble gravitational pull which is only one sixth as strong as the earth's gravity. Because there is no atmosphere to buffer heat, the temperature of the moon's surface varies between -180 degrees C and +110 degrees C.
Long thought to be devoid of water, it is now known that the moon harbours ice water in permanently shaded deep craters at its North and South poles.
The crust of the moon is 68 kilometres thick on average. Beneath the crust is a thick layer called the mantle, and there is probably a small core (340 kilometres radius and 2 per cent of moon's mass) at the centre. The moon's mantle is only partially molten, unlike that of the Earth which is entirely molten.
The surface of the moon is characterised by light mountainous regions interspersed with dark maria (huge impact craters later flooded by molten lava about three billion years ago). The "Man on the Moon" is formed from the arrangement of patches of these two terrains.
Much of the surface of the moon is covered with craters that resulted from impacts by meteors. The largest are about 200 kilometres across, the smallest are about one metre across. Most craters were formed between three and four billion years ago. Weathering on a planet like Earth would have smoothed these craters out long ago, but very little weathering occurs on the moon in the absence of atmosphere.
Most of the surface of the moon is covered with regolith, a mixture of dust and rocky debris produced by meteor impacts. Most rocks on the surface of the moon seem to be between three and 4.6 billion years old. The oldest rocks on Earth are rarely more than three billion years old. The moon therefore provides evidence of the early history of the solar system not available on Earth.
Unlike the Earth, the moon has no global magnetic field, but surface rocks show remnant magnetism indicating that the early moon may have had a global magnetic field.
How was the moon born? The most widely accepted theory at present is that when the solar system was very young, the Earth collided with a very large object (as big or bigger than Mars) and the moon formed from the material ejected from the Earth.
•William Reville is Associate Professor of Biochemistry and Director of Microscopy at UCC.