Kinetic energy. And yes, it will run out by dissipating energy through tidal interactions with the Sun. However, that will take a much longer time than would be required for Sun to become a red giant and destroy the Earth by engulfing it.
P.S. Such questions more properly belong in the General Astronomy forum...
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I am; therefore I think.
[This message has been edited by Boris (edited October 06, 1999).]
