Both same size and opposite sides (in the same orbit):
1. The gravity would not cancel. Gravity cannot 'cancel'.
2. The tides would be doubled, not cancelled. The tidal bulges occupy both sides of the earth from our only moon. The easiest way to see why this is so is to note that the moon pulls the oceans away from the earth on the nearside and pulls the earth almost as much away from the oceans on the opposite side. Hence, most places on the earth have two full tidal cycles in just over the earth's day.
2a. I stand corrected by Mr Nieveler, on the 'doubled tides' thank you.
3. I don't see a problem with doubled tides vis-a-vis the development of life, but something much worse would inevitably happen. The opposing Lagrange point is not a deep enough gravity well to hold that second moon, and it would drift off due to the effects of the gravity of other bodies in the solar system. As soon as one or the other had drifted near its pair, they would likely collide, giving earth rings like Saturn for a time. One might expect that a new moon would form but there would be no guarantee that it had to be the size of both together, or even as large as one original moon, because the impact debris would likey scatter into a wide range of orbits, some possibly escaping from the earth entirely, some falling to earth's surface. We might even end up with several moons, most of them smaller but still of significant size. This would be an interesting problem for a university student in astronomy to simulate numerically. It would make an interesting motion picture graphic for the general public..
1. The gravity would not cancel. Gravity cannot 'cancel'.
2. The tides would be doubled, not cancelled. The tidal bulges occupy both sides of the earth from our only moon. The easiest way to see why this is so is to note that the moon pulls the oceans away from the earth on the nearside and pulls the earth almost as much away from the oceans on the opposite side. Hence, most places on the earth have two full tidal cycles in just over the earth's day.
2a. I stand corrected by Mr Nieveler, on the 'doubled tides' thank you.
3. I don't see a problem with doubled tides vis-a-vis the development of life, but something much worse would inevitably happen. The opposing Lagrange point is not a deep enough gravity well to hold that second moon, and it would drift off due to the effects of the gravity of other bodies in the solar system. As soon as one or the other had drifted near its pair, they would likely collide, giving earth rings like Saturn for a time. One might expect that a new moon would form but there would be no guarantee that it had to be the size of both together, or even as large as one original moon, because the impact debris would likey scatter into a wide range of orbits, some possibly escaping from the earth entirely, some falling to earth's surface. We might even end up with several moons, most of them smaller but still of significant size. This would be an interesting problem for a university student in astronomy to simulate numerically. It would make an interesting motion picture graphic for the general public..
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