Hi everyone, This is my first post here and I am really sorry for that question, but I have found the answer nowhere. Consider a mass at the Earth's equator that is static in the Earth's referential during an entire day. Put the Earth at one of its equinoxes to simplify the problem. Then, at noon, the potential energy due to the sun of a mass m at that point is : E_p noon = -GMm/(distance Sun-Earth - radius of Earth) At midnight, E_p midnight = -GMm/(distance Sun-Earth + radius of Earth) Clearly, there is a difference in potential energy. By energy conservation, it should be balanced (by another kind of periodic energy variation). However, if the mass stand on solid ground, its rotation speed is the Earth rotation speed and it should be constant during the day (not going up and down) and therefore, the difference in kinetic energy is null. My questions : What balances this difference in potential energy? Is there a mistake in my reasoning?