ht coming from the
retreating edge will be shifted toward the red. And, by carefully noting
the amount of the shifting, the velocity of the planet's rotation can be
computed. This is what was done by Belopolski in the case of Venus, with
the result above noted.

Secondly, the theory that Venus rotates but once in the course of a
revolution finds but slight support from the doctrine of tidal friction,
as compared with that which it receives when applied to Mercury. The
effectiveness of the sun's attraction in slowing down the rotation of a
planet through the braking action of the tides raised in the body of the
planet while it is yet molten or plastic, varies inversely as the sixth
power of the planet's distance. For Mercury this effectiveness is nearly
three hundred times as great as it is for the earth, while for Venus it
is only seven times as great. While we may admit, then, that Mercury,
being relatively close to the sun and subject to an enormous braking
action, lost rotation until--as occurred for a similar reason to the
moon under the tidal attraction of the earth--it ended by keeping one
face always toward its master, we are not prepared to make the same
admission in the case of Venus, where the effective force concerned is
comparatively so slight.

It should be added, however, that no certain evidence of polar
compression in the outline of Venus's disk has ever been obtained, and
this fact would favor the theory of a very slow rotation because a
plastic globe in swift rotation has its equatorial diameter increased
and its polar diameter diminished. If Venus were as much flattened at
the poles as the earth is, it would seem that the fact could not escape
detection, yet the necessary observations are very difficult, and Venus
is so brilliant that her light increases the difficulty, while her
transits across the sun, when she can be seen as a round black disk, are
very rare phenomena, the latest having occurred in 1874 and 1882, and
the next not being due until 2004.

Upon the whole, probably the best method of settling the question of
Venus's rotation is the spectroscopic method, and that, as we saw, has
already given evidence for the short period.

Even if it were established that Venus keeps always the same face to the
sun, it might not be necessary to abandon altogether the belief that she
is habitable, although, of course, the obstacles to that belief would be
increased. Venus's orbit being so nearly circ