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 circular, and her orbital motion so nearly invariable, she has but a very slight libration with reference to the sun, and the east and west lunes on her surface, where day and night would alternate once in her year of 225 days, would be so narrow as to be practically negligible.