s greater than that which we
experience. When this planet is at its utmost distance from the sun the
intensity of solar radiation is even then more than four times greater
than the greatest heat which ever reaches the earth. But when Mercury,
in the course of its remarkable changes of distance, draws in to the
warmest part of its orbit, it is exposed to a terrific scorching. The
intensity of the sun's heat must then be not less than nine times as
great as the greatest radiation to which we are exposed.

These tremendous climatic changes succeed each other much more rapidly
than do the variations of our seasons. On Mercury the interval between
midsummer and midwinter is only forty-four days, while the whole year
is only eighty-eight days. Such rapid variations in solar heat must in
themselves exercise a profound effect on the habitability of Mercury.
Mr. Ledger well remarks, in his interesting work,[14] that if there be
inhabitants on Mercury the notions of "perihelion" and "aphelion," which
are here often regarded as expressing ideas of an intricate or recondite
character, must on the surface of that planet be familiar to everybody.
The words imply "near the sun," and "away from the sun;" but we do not
associate these expressions with any obvious phenomena, because the
changes in the distance of the earth from the sun are inconsiderable.
But on Mercury, where in six weeks the sun rises to more than double his
apparent size, and gives more than double the quantity of light and of
heat, such changes as are signified by perihelion and aphelion embody
ideas obviously and intimately connected with the whole economy of the
planet.

It is nevertheless rash to found any inferences as to climate merely
upon the proximity or the remoteness of the sun. Climate depends upon
other matters besides the sun's distance. The atmosphere surrounding the
earth has a profound influence on heat and cold, and if Mercury have an
atmosphere--as has often been supposed--its climate may be thereby
modified to any necessary extent. It seems, however, hardly possible to
suppose that any atmosphere could form an adequate protection for the
inhabitants from the violent and rapid fluctuations of solar radiation.
All we can say is, that the problem of life in Mercury belongs to the
class of unsolved, and perhaps unsolvable, mysteries.

It was in the year 1629 that Kepler made an important announcement as to
impending astronomical events. He had been s