two moons

[claymore]

If the Earth had two moons ,both roughly the same size and directly opposite each other, what would be the effect on the oceans. would there be no high and low tides or would we have twice as many?

[dundurn]

The tide pattern would be (largely?) unchanged but the tides would be higher (and lower) than at present.

[fo3nix]

I'd agree with the above.

It depends on the phases of the moons. Unless they're precisely the same as each other and moving at precisely the same speed and altitude, you're going to get some interference effects.

The same principle can be seen if you get two metal balls and hang them from pieces of string. Each one will be able to sway independantly. But if you connect them loosely with a spring (to simulate the effect of interference), you'll find that sometimes you get a damping effect where their amplitudes are reduced, and sometimes you get a constructive interference effect where their amplitudes are slightly increased. You're unlikely to see any large resonance effect I'd have thought, so the waves of the oceans would probably be a little larger and smaller than we currently have, alternating.

[wildwood]

As the Q says two moons directly opposite each other I would guess that means at all aspects (one is at 1/4 so the other one would be 3/4 etc.) so why would we not have twice as many tides then?

The highs and lows would be slightly different as now because of the changed gravitational forces.

[mibn2cweus]

. . . because there are already two each (both high and low) tides opposite each other associated with one moon.

Although the Sun's contribution to tide production is only about half (due to its much greater distance) that of the Moon, higher tides accompanying both New and Full Moons confirms that another gravitational body whether acting from the same or opposite side as the Moon results in more prominent tides.