MadSci Network: Earth Sciences

Re: How would multiple moons affect the tides?

Date: Tue Jan 26 18:21:53 1999
Posted By: Tye Morancy, Grad student/Teaching Asst. Physics and Radiology, UMASS Lowell/Radiation Tech. Harvard Univ. EH&S
Area of science: Earth Sciences
ID: 914551054.Es

   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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