Page:Astronomy for Everybody.djvu/181

To set forth what is known of the major planets we shall take them up in the order of their distance from the sun. The first planet reached will then be Mercury. It is not only the nearest planet to the sun, but much the smallest of the eight; so small, indeed, that, but for its situation, it would hardly be called a major planet. Its diameter is about two fifths greater than that of the moon, but, the volumes of bodies being proportional to the cubes of their diameters, it has about three times the volume of the moon.

It has far the most eccentric orbit of all the major planets, though, in this respect, it is exceeded by some of the minor planets to be hereafter described. In consequence, its distance from the sun varies between wide limits. At perihelion it is less than twenty-nine millions of miles from the sun; at aphelion it goes out to a distance of more than forty-three millions of miles. It performs its revolution around the sun in a little less than three months; to speak more exactly, in eighty-eight days. It therefore makes more than four revolutions in a year.

Performing more than four revolutions around the sun while the earth is performing one, we readily see that it must pass conjunction with the sun at certain regular though somewhat unequal intervals. To show