.
Alter erit tum Tiphys, et altera quae vehat Argo
Delectos heroas; erunt etiam altera bella,
Atque iterum ad Trojam magnus mittetur Achilles."

We have seen that the orbit of the earth is an ellipse, and that the sun
is situated at what is called the _focus_, a point not in the middle of
the ellipse, but rather towards one of its ends. Therefore, during the
course of the year the distance of the earth from the sun varies. The
sun, in consequence of this, is about 3,000,000 miles _nearer_ to us in
our northern _winter_ than it is in our northern summer, a statement
which sounds somewhat paradoxical. This variation in distance, large as
it appears in figures, can, however, not be productive of much
alteration in the amount of solar heat which we receive, for during the
first week in January, when the distance is least, the sun only looks
about _one-eighteenth_ broader than at the commencement of July, when
the distance is greatest. The great disparity in temperature between
winter and summer depends, as we have seen, upon causes of quite another
kind, and varies between such wide limits that the effects of this
slight alteration in the distance of the sun from the earth may be
neglected for practical purposes.

The Tides are caused by the gravitational pull of the sun and moon upon
the water of the earth's surface. Of the two, the moon, being so much
the nearer, exerts the stronger pull, and therefore may be regarded as
the chief cause of the tides. This pull always draws that portion of the
water, which happens to be right underneath the moon at the time, into a
heap; and there is also a _second_ heaping of water at the same moment
_at the contrary side of the earth_, the reasons for which can be shown
mathematically, but cannot be conveniently dealt with here.

As the earth rotates on its axis each portion of its surface passes
beneath the moon, and is swelled up by this pull; the watery portions
being, however, the only ones to yield visibly. A similar swelling up,
as we have seen, takes place at the point exactly away from the moon.
Thus each portion of our globe is borne by the rotation through two
"tide-areas" every day, and this is the reason why there are two tides
during every twenty-four hours.

The crest of the watery swelling is known as high tide. The journey of
the moon around the earth takes about a month, and this brings her past
each place in turn by about fifty minutes later each day, which is the