onderful changes in far away times; and can not help
seeing what labor is involved in the solution of all questions relating
to the same.<13>

The earth revolves around the sun in an orbit called an ellipse. This
is not a fixed form, but slowly varies from year to year. It is now
gradually becoming circular. It will, however, not become an exact
circle. Astronomers assure us that, after a long lapse of time, it will
commence to elongate as an ellipse again. Thus, it will continually
change from an ellipse to an approximate circle, and back again. In
scientific language, the eccentricity of, the earth's orbit is said to
increase and decrease.

Illustration of Earth's Orbit.--------------------

In common language we would state that the shape of the path of the
earth around the sun was sometimes much more elongated and elliptical
than at others. The line drawn through the longest part of an ellipse
is called the major axis. Now the sun does not occupy the center of this
line, but is placed to one side of it; or, in other words, occupies one
focus of the ellipse. It will thus be seen that the earth, at one time
during its yearly journey, is considerably nearer to the sun than
at others. The point where it approaches nearest the sun is called
_Perihelion,_ and the point where it reaches the greatest distance from
the sun is called its _Aphelion._ It will be readily seen that the more
elliptical its orbit becomes the greater will be the difference between
the perihelion and aphelion distance of the sun. At present the earth
is about three millions of miles nearer the sun in perihelion than in
aphelion. But we must remember the orbit of the earth is now nearly
circular. There have been times in the past when the difference was
about thirteen millions of miles. We must not forget to add, that the
change in the shape of the earth's orbit is not a regular increase and
decrease between well-known extremes. It is caused by the attraction of
the other planets. It has been calculated at intervals of ten thousand
years for the last million years. In this way it has been found that
"the intervals between connective turning points are very unequal in
length, and the actual maximum and minimum values of the eccentricity
are themselves variable. In this way it comes about that some periods
of high eccentricity have lasted much longer than others, and that the
orbit has been more elliptical at some epochs of high eccentricity than