Ok…we dealt with a moon beling blown up…Now what would happen if there were 2 moons…ala Fifth Element (which I liked…good in a bad b movie sorta way)
Extreme Tides? Super Super Eclipse? Could they infact drown the earth in crazy tidal surges?
It would really depend on the sizes and locations of the two moons.
Probably would cause some bizzare tidal effects, unless one of the moons was very small or very far away…
That moon in the 5th Element would likely have crashed into Earth, vaporizing the entire surface of our planet to a depth of a few hundred miles, and therefore terminating all life as we know it on this piece of rock. Notice how it was heading directly towards Earth before being stopped; without enough speed in the right direction to keep it in orbit, it would just drop down to Earth immediately like a big anvil…
Earth actually has two natural moons. The second one is just hard to see because it’s less than 100 feet in diameter. It was discovered only last year.
There’s something about it here.
3 miles across, but with an orbital period of over 700 years…weird.
Officially, as far as I can tell, it still seems to be classified as a Near Earth Asteroid, not a second moon.
Quote from that same website…
The story of 3753 Cruithne looks interesting, but could we stay focused on the OP? If I read it right, he wants to know what life on Earth would be like if we had a second moon of similar mass and size to Luna.
Could it share Luna’s orbit? For both moons to be stable, would its orbit have to be closer? Farther? More eccentric?
Would the tides be higher? Less predictable? Would we be able to surf?
How bright would the night sky be?
Would there be LaGrange points?
I suspect (but am too intimidated by the math to prove right now) that unless the moons were much farther away than Luna, two Lunar-mass moons would have stripped away too much of our atmosphere–there wouldn’t be enough left to support life as we know it. The tides would be much more complex than our real ones, at least until the oceans evaporated.
Assuming that we would retain enough atmosphere to live, the answers to the various questions would depend largely on the orbital periods (although the nocturnal illumination would depend on the albedo of the second moon). Assuming that the orbital periods were out of sync, you would get especially high tides when the moons were close to alignment, and lower than normal tides when they were on opposite sides of the Earth (although you would still get high tides on each side). At other times (e.g. moons 90 degrees out of phase), you would get slightly lower than normal high tides (I’m not sure what the low tides would do. If the moons had the same orbital period (unlikely at best), tides would follow one of the aforementioned patterns all the time (depending on the relative positions of the moons).
As for surfing, you would get the most impressive waves when the two moons were aligned. Alan Dean Foster wrote a mildly entertaining short story, “Surfeit”, about the monster waves generated when the three moons of Dis lined up during storm season.
I think there would still be stable Lagrange points–it would be based on the center of mass of the Earth-moons system relative to the sun. I’m not sure what effect it would have on the position of the points, though I suspect the second moon would move them farther away from Earth. Maybe one of the physics/planetary ballistics gurus can help. (Phobos, would you care to comment here?)
There’s my mathless analysis. Please feel free to blow it to bits if I goofed in my logic.
Balance: I believe that Larry Niven theory about a large moon being responsible for earth no being like venus is not generally accepted anymore.
It is true that Earth’s moon is unusual because it is so large (however, our sample size is pretty small).
The trouble with having two moons is that the moons will interact with each other. Let’s assume that we keep Luna, and are simply adding another moon, call it Selene. Well, another moon the size of Luna would be unlikely…where would it have come from? So, the other moon would have to be a pretty small body, perhaps a captured asteroid like Phobos and Deimos. However, the presence of Luna would tend to make the baby moon’s orbit unstable. Selene would either crash into Earth, be ejected, or form some sort of orbital resonance with Luna.
It would have to be quite a bit closer or farther out than Luna. Let’s assume it’s closer, cause that’s more cool. Well, it would probably make exactly 2 or 3 orbits for every Lunar orbit. Lots of the Gas Giant moons are in similar orbital resonances. I would suspect that Luna’s gravity would prevent any large eccentrity in Selene’s orbit, or it would be chucked out…so Selene might eclipse Luna every month.
Even if it was much closer it wouldn’t cause big tides if it was a Phobos/Deimos sized moon. These are tiny little things. But even so, it would add a third sine wave to the tide charts, and we’d get higher highs and lower lows and more medium mediums.
I think we’ll need one of the physicists here to chime in about the orbital mechanics of another Luna-sized object in orbit around the Earth. But I suspect that it would not be stable. Luna is already huge compared to the Earth (in comparison to other planets’ moons).
If it did work, the Earth’s tides would be very different, but still predictable.
The increased brightness of the night sky would make astronomy a bit more difficult. Obviously, the brightness would depend on the phases of the moons. Maybe someone can run the numbers to see the % increase in brightness for 2 full moons.
Yeah, so I’m one of the smallest moons in the solar system, and I’m probably just a captured asteroid, and I have a decaying orbit…you wanna make something of it? 
A resonance is the last place you would expect to see for a second moon, since the repeated gravitational influences in the same direction would cause one or both orbits to be unstable. It’s resonances from Saturn’s moons that cause the gaps in the rings (any ring particle that found itself in the gap would be in an unstable orbit), and resonances from Jupiter’s orbit cause similar gaps in the asteroid belt. If the two moons were at significantly different (non-resonant) distances, you wouldn’t have any orbital problems, and you would have Lagrange points corresponding to each one of them. If the orbital distances were similar, you’d probably end up in a Trojan configuration (an equilateral triangle) or in a dancing orbit like that of Earth and 3753 Cruithne (see prior links), where the two bodies approach each other until they get to a certain distance apart, whereupon the closer gets further away and the further one gets closer and they drift apart again. Two of Saturn’s moons are also in such an arrangement, but I don’t recall their names (Janus and something that starts with E).
As for tides and such, they’d be more complicated, but still predictable. If we assume that the two moons have similar composition (and hence density), then the tidal effects would be proportional to the cube of the apparent diameter, so the one that looked larger would be more significant (the Sun and the Moon have the same apparent diameter, but the Moon is more dense, so it produces larger tides).
The sun, being more massive but farther away, produces tides that are just about half what the moon produces. When the moon and sun line up, the tides are greatest (spring tide). When they are ninety degrees from each other, the tides are least (neap tides). If the sun were a little closer, or larger, and the solar tide was the same magnitude as the lunar tide, the neap tide would be almost no tide at all. During spring tide, you get 1 1/2 lunar tide. During neap tide, you get 1/2 lunar tide.
If you were to have a second moon of the same size and distance as our present moon, the effect would be similar, and depend upon the configuration. Sixty or 180 degrees apart would be fairly stable. If the moons were directly opposite each other, spring tide would be about 2 1/2 times, and neap tide would be 1 1/2. The tidal effect of moons 60 degrees apart would be 2&(&^3 !(@)% … xskorkskkks
I’d forgotten that Niven ever said anything about it, although he may have still influenced my logic. So, another large mass in a relatively close orbit wouldn’t have thinned the atmosphere much? Assuming that it wasn’t captured recently, of course–that would have catastrophic consequences of its own.
The only real way to put a Luna sized object in a stable orbit around Earth is to place it in a position much farther away from us than Luna, in which case it wouldn’t have a very large effect on us at all. We might get some more complicated tides, another object to stare at in the sky, and perhaps a longer day on Earth due to transfer of angular momentum.
Putting this new moon in any other location around us will likely result in it or Luna being ejected into space, or a huge collision between two of three bodies. In the former case, the resulting change in Earth’s orbit may turn us all into popsicles or shish kebabs, or affect nothing at all. In the later case, the collision could occur between the two moons or involve Earth and one moon. If the two moons hit, we would all get to see some incredible fireworks, after which the huge fireballs falling from the sky would make the dinosaurs’ extinction look like a joy ride. If the collision were between one of the moons and Earth, then all life on this planet would be terminated in a matter of minutes as the entire surface of Earth were vaporized to a depth of a few hundred miles. Future prospects for life on Earth would be bleak as all the precious water would now be lost in space. Some lucky bacteria may manage to hitch a ride on an asteroid and go on to harbor life in some other corner of the universe though…
I don’t know if Niven originated the idea that the moon was responsible for our (relatively) thin atmosphere, but I know he used it as a plot device in several stories. Other SF authors did as well, but I believe they have found that a large, close sattelite actually keeps the atmosphere thicker, because it creates a gas torus around the planet that keeps the air around (atmosphere leaves through outgassing but gets caught in the torus in the orbit of the sattelite, and sometimes slows down and returns to the planet).