Re: Planet rotation

[Gary Karshner <karshner@STMARYTX.EDU>]

Jim,
        You asked:
>      Is it possible for a planet not to rotate?
Relative to what? The fixed stars? It is possible but unlikely, as you
allude to in the rest of your note. The sun and other planets exert a tidal
pull that will tend to "lock" the planets rotation to their period. The
earth has done this to the moon, and the moon is trying to do the same to
us. During the age of dinosaurs the year was over four hundred days long,
but tidal friction has slowed down. If memory serves, Venus without a moon
presents a the same face to the Earth each times it passes us, and here the
tidal effect is far to small to account for this.  Friction in a moving
frame tends to make the object being acted on move at the frames speed.
        I hope this helps.
Gary

Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU