What if we had two moons?

Natorgator · Post by **Natorgator** » 2006-08-24 10:19am

So we all know that the moon has aided in evolutionary development by keeping our climate for changing rapidly as well as the tides.

But what if we had two moons, both the same size as the one we have now? What kind of effect would this have on the tides (especially if they were on opposite sides of the earth at any given time?)

Would evolution or civilization have progressed in a different manner?

General Trelane (Retired) · Post by **General Trelane (Retired)** » 2006-08-24 01:09pm

With one moon, we have two high tides and two low tides each day. Having two moons on opposites sides wouldn't change that, but I think that would be a highly unlikely scenario. More likely, the moons would be on different orbits, and their periods would be different. This would result in phases were their tidal forces work to together to cause nice, harmonious ocean tides, and longer phases were their tidal forces would interfere so as to cause a jumbled mess.

How this would affect evolution. . .I don't know. Many animals certainly rely on the tides for feeding. . .

CaptainChewbacca · Post by **CaptainChewbacca** » 2006-08-24 03:21pm

I think things would be largely the same, except night vision in animals would be even poorer because you'd have two lights in the night sky.

Max · Post by **Max** » 2006-08-24 04:05pm

Wouldn't there be a noticeable difference in gravity?

CaptainChewbacca · Post by **CaptainChewbacca** » 2006-08-24 04:16pm

^Probably not. What makes up the moon is mostly lightweight (relatively) elements. I mean, you don't suddenly feel lighter when the moon is directly overhead, do you?

darthkommandant · Post by **darthkommandant** » 2006-08-24 04:22pm

Is this kind of system stable though? Can Terra actually have 2 moons without them colliding of crashing down?

CaptainChewbacca · Post by **CaptainChewbacca** » 2006-08-24 04:42pm

darthkommandant wrote:Is this kind of system stable though? Can Terra actually have 2 moons without them colliding of crashing down?

A while back they did some modeling on the original formation of the moon. In 80 or so computer simulations, there was a single moon. In about 10 of them, a second, inner moon formed at about 140,000 miles up. Unfortunately, in none of those simulations was it stable for more than a few hundred million years. Colliding isn't an issue, as you wouldn't have two bodies forming on the same orbit.

Max · Post by **Max** » 2006-08-24 04:49pm

CaptainChewbacca wrote:^Probably not. What makes up the moon is mostly lightweight (relatively) elements. I mean, you don't suddenly feel lighter when the moon is directly overhead, do you?

No, but I imagine if one moon has an effect on things, such as tide, that a second one may duplicate the effect. We may not physically feel the effect, but there IS an observable one. Maybe someone that studies astronomy can add some input, as I'm just guessing obviously.

CaptainChewbacca · Post by **CaptainChewbacca** » 2006-08-24 04:55pm

Max wrote:
CaptainChewbacca wrote:^Probably not. What makes up the moon is mostly lightweight (relatively) elements. I mean, you don't suddenly feel lighter when the moon is directly overhead, do you?
No, but I imagine if one moon has an effect on things, such as tide, that a second one may duplicate the effect. We may not physically feel the effect, but there IS an observable one. Maybe someone that studies astronomy can add some input, as I'm just guessing obviously.

In the two-moon scenarios I mentioned above, you start with the same amount of orbiting debris you do for a single moon, so tides would likely be less severe in general, when the moons line up the fact that there's a closer moon would make tides a bit greater.

General Trelane (Retired) · Post by **General Trelane (Retired)** » 2006-08-24 05:01pm

The reason that the moon has an observable effect on our oceans is two-fold:

1. The ocean is large and unbounded. You do not see any tidal effects on a backyard pool.

2. All gravitational forces decrease with the inverse of the square of the distance.

So because the ocean is large enough, the variation in the moon's gravitational attraction throughout the expanse of the ocean is what causes the tides.

Actually, the sun also has a tidal effect on our oceans, but it's small compared to that of our moon. But at new and full moons, the two complement each other, and we do get higher high tides.

The highest tides of all would occur during a new moon at perigee and perihelion. Adding a second moon would only complicate matters and perhaps cause more extreme variations in the tides.

Talanth · Post by **Talanth** » 2006-08-24 06:18pm

I heared once, I cant remember from where, that the moon helps in keeping the planitary wobble on a resonable level. Without the moon the angle of precesion would be so great, and the resulting seasons so severe, that life probably couldn't exist. What two moons would cause I don't know.

Guardsman Bass · Post by **Guardsman Bass** » 2006-08-24 07:59pm

Talanth wrote:I heared once, I cant remember from where, that the moon helps in keeping the planitary wobble on a resonable level. Without the moon the angle of precesion would be so great, and the resulting seasons so severe, that life probably couldn't exist. What two moons would cause I don't know.

That's not completely correct. What it does is prevent any major changes in axial tilt. Mars, for example, can vary from 0-90 degree tilt, whereas the moon keeps Earth's tilt in between 21 and 24.

CoyoteNature · Post by **CoyoteNature** » 2006-08-24 08:25pm

I wonder how plate tectonics would be affected with two moons?

If at all?

CaptainChewbacca · Post by **CaptainChewbacca** » 2006-08-24 08:58pm

^Tectonics wouldn't really be affected. They're too heavy and rigid, and the moon is too far away.

CoyoteNature · Post by **CoyoteNature** » 2006-08-24 09:25pm

Really, but I was also thinking about the formation of the two moons, we know that a single Mars size impactor created our current moon, so either in this that impactor created two moons or there were two impact events.

If the two maybe that would have altered plate tectonics.

I was wondering about the gravitational effects also, thanks.