Freethought & Rationalism Archive

Vorkosigan · 09-22-2002, 09:53 PM

But...the Earth DOES have three moons, including one captured quite recently. The other two are very small. So Earth is such as planet as you've described....

<a href="http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html" target="_blank">here</a>

I can't find a reference for the newest discovery....

[ September 22, 2002: Message edited by: Vorkosigan ]

lpetrich · 09-22-2002, 10:06 PM

Here's an approximate formula for tide height:

h ~ R*((m/a^3)/(M/R^3))

R = Earth's radius
M = Earth's mass
a = object's distance
m = object's mass

Applied to the Earth and the Moon, it yields 36 cm or a little over 1 foot. Likewise, for the Earth and the Sun, it yields 15 cm or 0.5 feet.

However, tides in many places are much higher than that due to the oceans being pulled horizontally as well as vertically, which pulls them against coastlines.

I hope that this is at least somewhat helpful.

The 1/a^3 comes about because the tidal force is the difference in the force of gravity in different locations. This is also why there are two tides a day; the side away from the Moon gets left behind as the Earth gets pulled toward the Moon.

crocodile deathroll · 09-22-2002, 10:54 PM

Quote:

Originally posted by Coragyps:
Not speaking as an expert....
I would think that you could make the tides do nearly anything you wanted by picking the periods of the large moons. If all three lined up, even on opposite sides of the planet, the tides would be additive and maybe huge, and when the three were (for instance) 120 degrees apart, tides would be hardly noticeable. I don't think you could have moons with integral ratios of periods, like moon A seven days, B 14, and C 21, as those resonances wouldn't give stable orbits. But 7, 13, and 19 or some such might be OK, and would still give alignments every so often.

The faraway moon could probably be ignored as far as tides go; I'll bet that the calculations with even three moons are the kind that mathematicians call "intractable."

And I want to read the story when it's finished!

Woudn't it be far simpler if the present moon was 4 times more massive. Or even if the Earth was a binary planet like the many examples that are observed of binary stars. The influence then on tides tides would be stupendous

ohwilleke · 09-23-2002, 12:35 PM

My intuition is that you are going to have to have either nested orbits (a la planets around sun or moons around Jupiter) with some moons dominant in their effects over the others, or identical out of phase orbits, for a system that is stable in the long run. Orbits which are close to each other, but not identical are going to tend towards collision -- because the gravitation forces between the moons themselves are naturally going to draw the orbits closer to each other.

The distant small moon will have a negligable effect gravitationally. The other three moons, being equal in size, will have an impact proportionate to their distance.

Also, the tides should on the whole be less bad in a multiple moon system than in a single moon system of comparable size. Basically, a single large moon is going to produce a big up and a big down in the tide. Four mooons of equal mass will never produce more of a tide than that (if they are all in alignment) and will often (more often the larger the number of the moons) cancel each other out somewhat (almost entirely) if they are of comparable sizes.

If they are not of comparable sizes, you will get a big major moon rhythm, that will be more or less muted by the next biggest moon, that will be slightly adjusted by the next moon, and so on.

You would probably get seas that are on the whole a lot more placid than Earths, with infrequent, perhaps every several years, exceptional surges. (Which may materialize without a lot of warning). Probably the biggest two moons would control the main big surges, so you'd have to figure out how often they were aligned or opposed (alignments and oppositions of a two moon system should occur with approximately the same frequency as eclipses do on Earth), and then the minor moons would slightly damp or boost these peaks on far less frequent cycles on the order of decades.

The other effect that the three large moons would have if they weren't all in the same plane of orbit, is that they would probably induce a wobble into the main planets axis, making seasons far less reliable. One year might have a ten month summer, and another might have a two month summer, with seasons sometimes changing slowly, and sometimes lurching dramatically from one to another.

[ September 23, 2002: Message edited by: ohwilleke ]

Abacus · 09-23-2002, 01:03 PM

Not being a geologist myself, I may be off base. But, wouldn't the presence of three large moons lead to more volcanic activity?

Dark Jedi · 09-23-2002, 06:11 PM

related info:

co-orbiting bodies as temporary moons. Maybe that could work in to your storyline better?

Discussion in <a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=57&t=000509" target="_blank">this</a> thread.

Perchance · 09-24-2002, 11:50 AM

Thanks for the information, everyone!

I'm still not sure which set-up to use in my story yet, but at least now I know there's more than one option.

Thank you!

-Perchance.

crocodile deathroll · 09-26-2002, 06:07 AM

Quote:

Originally posted by crocodile deathroll:
An even simpler solution it to position our present moon in an orbit about twice as close as it is now. From my earlier research on the suject I read somewhereit was much much closer to Earth a few billion years ago.

[ September 26, 2002: Message edited by: crocodile deathroll ]

09-22-2002, 09:53 PM	#11
Vorkosigan Contributor Join Date: Jan 2001 Location: Barrayar Posts: 11,866	But...the Earth DOES have three moons, including one captured quite recently. The other two are very small. So Earth is such as planet as you've described.... <a href="http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html" target="_blank">here</a> I can't find a reference for the newest discovery.... [ September 22, 2002: Message edited by: Vorkosigan ]</p>

09-23-2002, 12:35 PM	#14
ohwilleke Veteran Member Join Date: Jun 2000 Location: Denver, Colorado, USA Posts: 4,834	My intuition is that you are going to have to have either nested orbits (a la planets around sun or moons around Jupiter) with some moons dominant in their effects over the others, or identical out of phase orbits, for a system that is stable in the long run. Orbits which are close to each other, but not identical are going to tend towards collision -- because the gravitation forces between the moons themselves are naturally going to draw the orbits closer to each other. The distant small moon will have a negligable effect gravitationally. The other three moons, being equal in size, will have an impact proportionate to their distance. Also, the tides should on the whole be less bad in a multiple moon system than in a single moon system of comparable size. Basically, a single large moon is going to produce a big up and a big down in the tide. Four mooons of equal mass will never produce more of a tide than that (if they are all in alignment) and will often (more often the larger the number of the moons) cancel each other out somewhat (almost entirely) if they are of comparable sizes. If they are not of comparable sizes, you will get a big major moon rhythm, that will be more or less muted by the next biggest moon, that will be slightly adjusted by the next moon, and so on. You would probably get seas that are on the whole a lot more placid than Earths, with infrequent, perhaps every several years, exceptional surges. (Which may materialize without a lot of warning). Probably the biggest two moons would control the main big surges, so you'd have to figure out how often they were aligned or opposed (alignments and oppositions of a two moon system should occur with approximately the same frequency as eclipses do on Earth), and then the minor moons would slightly damp or boost these peaks on far less frequent cycles on the order of decades. The other effect that the three large moons would have if they weren't all in the same plane of orbit, is that they would probably induce a wobble into the main planets axis, making seasons far less reliable. One year might have a ten month summer, and another might have a two month summer, with seasons sometimes changing slowly, and sometimes lurching dramatically from one to another. [ September 23, 2002: Message edited by: ohwilleke ]</p>

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09-22-2002, 10:06 PM	#12
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Here's an approximate formula for tide height: h ~ R*((m/a^3)/(M/R^3)) R = Earth's radius M = Earth's mass a = object's distance m = object's mass Applied to the Earth and the Moon, it yields 36 cm or a little over 1 foot. Likewise, for the Earth and the Sun, it yields 15 cm or 0.5 feet. However, tides in many places are much higher than that due to the oceans being pulled horizontally as well as vertically, which pulls them against coastlines. I hope that this is at least somewhat helpful. The 1/a^3 comes about because the tidal force is the difference in the force of gravity in different locations. This is also why there are two tides a day; the side away from the Moon gets left behind as the Earth gets pulled toward the Moon.

09-23-2002, 01:03 PM	#15
Abacus Veteran Member Join Date: Mar 2002 Location: South Dakota Posts: 2,214	Not being a geologist myself, I may be off base. But, wouldn't the presence of three large moons lead to more volcanic activity?

09-23-2002, 06:11 PM	#16
Dark Jedi Veteran Member Join Date: Oct 2001 Location: Afghanistan Posts: 4,666	related info: co-orbiting bodies as temporary moons. Maybe that could work in to your storyline better? Discussion in <a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=57&t=000509" target="_blank">this</a> thread.

09-24-2002, 11:50 AM	#17
Perchance Veteran Member Join Date: May 2002 Location: Kentucky Posts: 1,059	Thanks for the information, everyone! I'm still not sure which set-up to use in my story yet, but at least now I know there's more than one option. Thank you! -Perchance.

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