| I didn't go to Caltech ;) It can also be described via a conservation law. If the Earth were frozen solid, and the Moon+Earth system started up to something like we have now, then they would be in a stable orbit. Next, thaw the water. If only one lobe forms then there's a violation of conservation of angular momentum. To really see the problem, use the planet "H2O", made entirely of ice, instead of Earth. Once the ice thaws, which water decides to form the single lobe? If all of it flows towards the Moon then it wouldn't be a stable orbit. There can't be zero lobes because the system is asymmetric around the barycenter. If everything is in equilibrium then the result needs to be axially symmetric, giving two asymmetric lobes; one towards and one away from the Moon. (The asymmetry is due to the size of the Earth vs. the distance to the Moon. It's about 60 radii away, so not that asymmetric, but the near-side tides will be higher. As another example, a micro black hole orbiting very close to the water surface will only have noticeable local effects.) Of course, in real life the system isn't in equilibrium. The Earth isn't tidally locked to the Moon, the orbits aren't perfectly circular, the bodies wobble and aren't spherical, etc. and the sea isn't deep enough for this lobe wave to travel, so it isn't true that there are two bulges. http://physics.stackexchange.com/a/121858 gives a lovely explanation and http://www.lhup.edu/~DSimanek/scenario/tides.htm works through many "Tidal Misconceptions". As a result, some places have more than two tides, like http://www.southamptonvts.co.uk/live_information/tides_and_w... , with High Water, Second Tide, and Low Water. P.S. I hope I got that correct! The last time I did this sort of analysis was 20 years ago. And personally, my intuition is based on the stretching force interpretation. One of the mechanics problems I worked out then was to explain why tidal forces radially align the long axis of a spacecraft. |