It would be beyond our scope to attempt to explain the complete theory,
but we may mention one obvious factor which must be taken into account.
Since the moon, by its gravitational attraction, produces tides, we
should expect that the sun, whose gravitational attraction is so much
stronger, should also produce tides and, we would suppose at first
sight, more powerful tides than the moon. But while it is true that the
sun produces tides, it is not true that they are more powerful than
those produced by the moon. The sun's tide-producing power is, as a
matter of fact, less than half that of the moon. The reason of this is
that _distance_ plays an enormous rôle in the production of tides. The
mass of the sun is 26,000,000 times that of the moon; on the other hand
it is 386 times as far off as the moon. This greater distance more than
counterbalances its greater mass, and the result, as we have said, is
that the moon is more than twice as powerful. Sometimes the sun and moon
act together, and we have what are called spring tides; sometimes they
act against one another, and we have neap tides. These effects are
further complicated by a number of other factors, and the tides, at
various places, vary enormously. Thus at St. Helena the sea rises and
falls about three feet, whereas in the Bay of Fundy it rises and falls
more than fifty feet. But here, again, the reasons are complicated.
§ 17