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Originally Posted by The Bad Astronomer
You get tides even if the system is not rotating. Let the Earth fall into the Sun, and you still get differential tides across the Earth, even when the net angular momentum is zero.
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As far as I see there is no real difference at all between the DO theory and the DF (Differential Forces) theory in the case where a non-rotating Earth is simply falling into the Sun. In that case, the tidal bulge on the far side really is being left behind by the rest of the Earth, while the bulge on the near side is outstripping the rest of the planet. It's the way I envisaged the tides in my
OP.
But imagine, for a moment, what would happen in such a case if the Earth were prevented from falling. For example, imagine there is a spike driven through the centre of the Earth and protruding from the poles; now imagine that a giant being took hold of the ends of that spike and held the Earth above the Sun. What sort of tidal bulges would you get? There would still be a bulge on the near side as the Earth tried to slip off the spike and into the Sun, but there would be no bulge on the far side. The Earth would be pear-shaped. It would be like an ice-cream lolly that is being held horizontally to the ground, and is starting to melt and slide off the stick!
In this (admittedly fantastic) scenario, the far side of the Earth does not bulge away from the Sun
because it is not moving through the Sun's gravitational field with the necessary velocity to climb up the gravitational gradient. In fact, it's not moving at all. It's like a rocket sitting on the launch-pad: it ain't going nowhere!
But those same differential forces that produced the normal tidal bulges still apply. In this case they don't produce the normal tides because the Earth's CoM is no longer in free fall. A person at the centre of the Earth would feel the Sun's gravity. But someone on the far side of the Earth would still feel a weaker force than someone at the CoM!