What would the effect of 2 moons be on Earth?

Question

Supposing Earth had a second moon of comparable size as our currect moon, what effects would that have on Earth?

Here's a couple scenarios:

(1) The second moon on exact opposite side of the planet, both moons had the same orbits.

(2) The second moon directly behind our moon, an equal distance from that moon as the first has to Earth, and both having the same orbit.

(3) The second moon directly behind our moon, an equal distance from that moon as the first has to Earth, but in one case has an orbit 2x as long as our moon, and in another case half as long.

As a bonus, what would happen if instead of a moon it was a planet with oceans of it's own? How would Earth and moon affect its oceans?

And just to clear it up before it's asked, this is purely for my own interest :]

Answer 1

None of your "both having the same orbit" scenarios would work. They wouldn't be stable - two objects cannot share the same orbit. Gravitational influences from Earth, the sun, the elliptical nature of Earth's orbit and the moon's orbit would either destroy one moon, make them crash into each other, or throw one into a different orbit (from orbital and kinetic physics).

But if we assume 2 moons, same mass but different orbits, then we have a few possibilities. We'll call our current moon Moon (capital M) and the 2nd moon lower case.

If the moon is twice the distance of the Moon, but the same mass, the tidal influence of the moon will be one quarter of the Moon (inverse square law).
We would have 2 high and 2 low tides each day, but they would change with the Moon and moon phases - some days (when both Moon and moon are full or new at the same time) the high and low tides would be at an extreme.
That would apply whether the moon's orbit was half or twice as long as the Moon's.
The actual pattern and interactions of the tides would be complex, since you also have to take into account the sun's effect (its less than the Moon but still measurable).

If the moon was a planet with oceans, it would depend on its distance from the Earth and from the Moon.
And it would depend on the mass of the planet. Too small (like the Moon) and it couldn't keep an ocean (it would evaporate and be lost to space due to insufficient gravity to hold it).
If it was a planet even half the mass of Earth, its most likely that the planet would orbit the Earth/Moon system as a whole. I don't see any way a planet half the mass of the Earth could orbit the Earth inside the Moon's orbit without tossing out or destroying our Moon.
So the tides on the planet would be controlled by the total gravitational effect of the entire Earth/Moon system as one point.
And the planet would influence the tides on Earth in tune with the planet's orbit and distance (inverse square law again).
I don't have the equations handy to calculate the actual gravitational effect if the planet was half the mass of the Earth and twice as far as the Moon (sorry, any physics or math folks out there could probably help).

Answer 2

Most of your scenarios are not physically possible. There are only a few configurations of mutually gravitationally interacting bodies that are stable.

In the first case, for example, the attraction of the two moons toward each other would cause them to crash into Earth.

There may be a stable configuration with moons at 1 and 2 LD (lunar distance) away, but you can't have arbitrary orbital speeds. The speed and distance of an orbit must be such that the gravitational attraction to the center of the orbit provides the precise amount of centripetal force to keep the orbiting body in place, so there is at most one combination of such orbits. (Google Kepler's laws) In this case the tidal effect of the second moon would be only 1/4th of the current moon, but there would be times when the gravitational forces aligned and our ocean tides would be more extreme than they are now.

Answer 3

1. Tides are going to affect for sure 2. Lunar month will be of 14 days approx. For every 7 days you can see full moon and new moon 3. Earth gravity will not be affected 4. Earth rotation will not be affected 5. Number of lunar and solar eclipse will be double what we see now 6. In an eclipse, you can see sun, earth and two moons all in same line and in no night you can see both the moons together (unless the second moon is directly placed opposite in the orbit)

Answer 4

Effects:

1.) all calendars would have to be junked.

2.) Night time observers would have a really hard time planning to go out at "night" since two moons would light up the night sky and make it day time- almost.

3.) In the midst of a high tide, we would have a sudden low tide.

4.) all farmer's almanacs would have to be rewritten.

5.) A planet does not orbit around a planet, by definition. So that question is flawed. You mean a really big moon. Well, i guess NASA would have to plan another Moon Landing then.

Answer 5

Hmmm that's an interesting question!
The first one would destroy the tidal flow-there would be no more tides. Oceans would be only a bit more active than lakes. The second one wouldn't touch tides. The third one-both cases would create the same type of tidal interruption, but their effect (since they'd be so far away) would be minimal. Tides would probably shrink in intensity. And for your awesomely awesome bonus question-I'm sorry, I'm going to need more information-how big would this planet be? Would it be as big as the moon or as big as the earth? And also how far away would it be?

Answer 6

Dang good question!!!
It probably doesn't serve any purpose other than discussion but it is an interesting question!