At the beginning, when the moon split off from the earth, it obviously
must have shared the earth's rotation. It flew round the earth in the
same time that the earth rotated, that is to say, the month and the day
were of equal length. As the moon began to get farther from the earth,
the month, because the moon took longer to rotate round the earth, began
to get correspondingly longer. The day also became longer, because the
earth was slowing down, taking longer to rotate on its axis, but the
month increased at a greater rate than the day. Presently the month
became equal to two days, then to three, and so on. It has been
calculated that this process went on until there were twenty-nine days
in the month. After that the number of days in the month began to
decrease until it reached its present value or magnitude, and will
continue to decrease until once more the month and the day are equal. In
that age the earth will be rotating very slowly. The braking action of
the tides will cause the earth always to keep the same face to the moon;
it will rotate on its axis in the same time that the moon turns round
the earth. If nothing but the earth and moon were involved this state of
affairs would be final. But there is also the effect of the solar tides
to be considered. The moon makes the day equal to the month, but the sun
has a tendency, by still further slowing down the earth's rotation on
its axis, to make the day equal to the year. It would do this, of
course, by making the earth take as long to turn on its axis as to go
round the sun. It cannot succeed in this, owing to the action of the
moon, but it can succeed in making the day rather longer than the month.
Surprising as it may seem, we already have an illustration of this
possibility in the satellites of Mars. The Martian day is about one
half-hour longer than ours, but when the two minute satellites of Mars
were discovered it was noticed that the inner one of the two revolved
round Mars in about seven hours forty minutes. In one Martian day,
therefore, one of the moons of Mars makes more than three complete
revolutions round that planet, so that, to an inhabitant of Mars, there
would be more than three months in a day.