Between midsummer (of the northern hemisphere) and midwinter our planet
draws 3,000,000 miles nearer the sun, but the change occupies six
months, and, at the earth's great average distance, the effect of this
change is too slight to be ordinarily observable, and only the
astronomer is aware of the consequent increase in the apparent size of
the sun. It is not to this variation of the sun's distance, but rather
to the changes of the seasons, depending on the inclination of the
earth's axis, that we owe the differences of temperature that we
experience. In other words, the total supply of heat from the sun is not
far from uniform at all times of the year, and the variations of
temperature depend upon the distribution of that supply between the
northern and southern hemispheres, which are alternately inclined
sunward.

But on Mercury the supply of solar heat is itself variable to an
enormous extent. In six weeks, as we have seen, Mercury diminishes its
distance from the sun about one third, which is proportionally ten times
as great a change of distance as the earth experiences in six months.
The inhabitants of Mercury in those six pregnant weeks see the sun
expand in the sky to more than two and a half times its former
magnitude, while the solar heat poured upon them swiftly augments from
something more than four and a half times to above eleven times the
amount received upon the earth! Then, immediately, the retreat of the
planet begins, the sun visibly shrinks, as a receding balloon becomes
smaller in the eyes of its watchers, the heat falls off as rapidly as it
had previously increased, until, the aphelion point being reached, the
process is again reversed. And thus it goes on unceasingly, the sun
growing and diminishing in the sky, and the heat increasing and
decreasing by enormous amounts with astonishing rapidity. It is
difficult to imagine any way in which atmospheric influences could