This immense variation of distance is emphasized by the rapidity with which it takes place.

Mercury's periodic time, i.e., the period required for it to make a single revolution about the sun--or, in other words, the length of its year--is eighty-eight of our days.

In just one half of that time, or in about six weeks, it passes from aphelion to perihelion; that is to say, in six weeks the whole change in its distance from the sun takes place.
In six weeks Mercury falls 14,000,000 miles--for it _is_ a fall, though in a curve instead of a straight line--falls 14,000,000 miles toward the sun! And, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede.

At the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward.
Of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than Mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall! Next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination.
First, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance.

The earth's distance from the sun being 93,000,000 miles, while Mercury's is only 36,000,000, it follows, to begin with, that Mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does.