It is true that we have a slight effect of this kind on the earth.  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 equalize the effects of such violent changes, or any adjustments in the physical organization of living beings that could make such changes endurable.

But we have only just begun the story of Mercury’s peculiarities.  We come next to an even more remarkable contrast between that planet and our own.  During the Paris Exposition of 1889 a little company of astronomers was assembled at the Juvisy observatory of M. Flammarion, near the French capital, listening to one of the most surprising disclosures of a secret of nature that any savant ever confided to a few trustworthy friends while awaiting a suitable time to make it public.  It was a secret as full of significance as that which Galileo concealed for a time in his celebrated anagram, which, when at length he furnished the key, still remained a riddle, for then it read:  “The Mother of the Loves imitates the Shapes of Cynthia,” meaning that the planet Venus, when viewed with a telescope, shows phases like those