Professor G. H. Darwin has calculated the relative effect of tidal retardation by the Sun on each of the several planets, that upon the Earth being taken as unity, with the accompanying result:—
| Planet. | Number to which Tidal Retardation is Proportional. | |
| Mercury | 1000 | .(?) |
| Venus | 11 | .(?) |
| Earth | 1 | . |
| Mars | .89 | |
| Jupiter | .00005 | |
| Saturn | .000020 to .000066 | |
Supposing, then, all the bodies to have started in the race for rotary retardation at the same time, the isochronism of rotation and revolution of Mercury is what was to have been expected. For the previously known facts were: first, that the Moon showed this state of things. Now the relative tide-raising effect in a given time of the Earth on the Moon is 2374; that of the Sun on the Earth being unity. Second, that Iapetus did the same; for this satellite is always much brighter on the western side of Saturn than on the eastern. Such a periodic change of brilliancy would be accounted for by isochronism of rotation and revolution. Now the relative tide-raising effect of Saturn on this satellite is 28.
