|MadSci Network: Earth Sciences
Hi Julia, That's an interesting question, since our moon has so much of an effect on our world, especially as seen in the changing tides in our major bodies of water. As an aside, there are much more important things about moons than the number of moons in a certain sense. How big the moon is and its relative size to the planet which it orbits are important factors. The earth's moon is very special in this case. Most of the moons in the solar system are much smaller than our own moon. There are only about 6 others as large as our moon. These moons are divided among Jupiter (4), Saturn (1), and Neptune (1). Jupiter, Saturn, and Neptune are all very large planets. Our planet is the only case we know about which has a large moon associated with a small planet with the exception of Pluto which is a much smaller planet than the Earth. Although Pluto's moon is much smaller than our moon, it is perhaps larger relative to the size of the planet. Pluto, however, is so far away and so small that we really do not know much about it. A planet like Mars has two moons, but they are both so small that they affect the planet very little. Even a large moon does not have large effects on a planet in the general sense. However, the effects of our moon, though subtle, are very important ones. Our moon, for example, contributes approximately double the tidal effects of the sun's on the earth. The cycle of ocean tides on the earth are very complicated and yet simple and wondrous. The fact that there are tidal zones on the beaches of the world had an enormous effect on the way that life got started and evolved on earth. There are also tidal effects, less visible, but equally important, on the earth's atmosphere, which influence the weather and climate. To have a large tidal effect on a planet, a moon must be not only large, but also reasonably close. For instance, Saturn's 1 large moon and Jupiter's 2 largest moons are too far away to have a large tidal effect. Only Neptune's large moon, Triton, and Jupiter's two inner moons, Io and Europa, would be in an ideal position to exert a large enough tidal influence close to the extent that our own moon does. In addition to relative size and there are additional factors such as orbital period and composition which have valuable characteristics for life on earth. If a large planet has a moon close by, then it will travel around the planet in a single day, or perhaps even less. Our large moon is close enough to have a big tidal influence, but our planet is small enough that the moon takes 28 days to complete an orbit, or 30 days to complete a cycle of phases. The two instances provide for some wonderful consequences. Most biological systems work in conjunction with a daily rhythm of light and dark, or a yearly rhythm of the seasons. The lunar cycle provides a more subtle trigger, that spans the gap nicely: 30 times longer than a day, but 12 times shorter than a year. Some sea creatures have behaviour cycles that are triggered by the bright nights of the full moon, for example. The reflectance of a moon is also very important. Our moon is made of rocky material, and is quite black. It reflects only about 9% of the visible light it receives. An icy moon like Jupiter's Europa reflects 39%, that is, it is much brighter. If our moon were made of ice instead of rock, that might make full moonlight much brighter, certainly bright enough to read by. It would be less soft and subtle and romantic. And it might even be hard to get to sleep. The most remarkable thing of all about our moon is that the size of its disk (the apparent circular disk we see facing us when we look at the moon...) is almost exactly the same size as the sun's disk. That might not affect science much, but it affects culture and history enormously through the occurrences of eclipses. So to sum up, the number of moons can either have a large influence or an insignificant effect depending on the above factors. Obviously, we would want one of them to be close enough and large enough to sustain such things as these nice cyclic tidal influences to sustain some of the ocean's ecosystems and creatures. You could proceed to add many more moons that were further away and smaller to exert smaller effects that would not upset the system too much. The existence of more moons would also effect culture and society through the ages as well if you think about it. The moon also is moving away from the earth a little every year and its cycle is slowing down. This means that many millions of years down the road the moon's period will increase from approximately a month to as much as 50-60 days. The moon used to be closer to the earth and had a shorter orbital cycle. This process happens because earth and our moon orbit each other and as time goes on energy and angular momentum must be conserved. These quantities are lost by our planet's spin and given to the moon, increasing its distance and orbital period. Its a fascinating process and worth reading up on. Thanks for the question, I hope my answer helped you... Tye Morancy
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