Being much nearer the sun than the earth is, Mercury can be seen by us only in the same quarter of the sky where the sun itself appears.  As it revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night.  When seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it.  When it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays.  Consequently, except with the instruments of an observatory, which are able to show it in broad day, Mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun.

The nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and Mercury, being the nearest to the sun of all the planets, is by far the swiftest footed among them.  But its velocity is subject to remarkable variation, owing to the peculiar form of the orbit in which the planet travels.  This is more eccentric than the orbit of any other planet, except some of the asteroids.  The sun being situated in one focus of the elliptical orbit, when Mercury is at perihelion, or nearest to the sun, its distance from that body is 28,500,000 miles, but when it is at aphelion, or farthest from the sun, its distance is 43,500,000 miles.  The difference is no less than 14,000,000 miles!  When nearest the sun Mercury darts forward in its orbit at the rate of twenty-nine miles in a second, while when farthest from the sun the speed is reduced to twenty-three miles.

Now, let us return for a moment to the consideration of the wonderful variations in Mercury’s distance from the sun, for we shall find that their effects are absolutely startling, and that they alone suffice to mark a wide difference between Mercury and the earth, considered as the abodes of sentient creatures.  The total change of distance amounts, as already remarked, to 14,000,000 miles, which is almost half the entire distance separating the planet from the sun at perihelion.  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.