SP-345 Evolution of the Solar
FIGURE 9.71.- Periods of axial
rotation for some asteroids and some of the planets in relation to
their masses. (From Alfvén, 1964.) The rotation period of
Pluto is not well known and the rotation periods of Mercury and Venus
are influenced by resonance effects; these three planets are thus not
represented in the figure. The value of rotation period for the Earth
is that prior to capture of the Moon. Data for asteroids is taken
from table 9.7.1 and data for the planets from table 21.1. From the
graph one concludes that spin period is not a function of mass.
indeed, most of the spin periods all fall within a factor of two of 8
hr. We refer to this similarity of periods of rotation as the law of
isochronous rotation or the isochronism of spins.
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