In speaking of the orbits, I do not mean that the planets have visible marked pathways through the heavens, any more than a swallow has a visible pathway through the sky, or a ship a marked pathway through the sea. Yet each planet has his own orbit, and each planet so distinctly keeps to his own, that astronomers can tell us precisely whereabouts in the heavens any particular planet will be, at any particular time, long years beforehand. There is also another mode of grouping the planets, besides dividing them into superior and inferior planets. By this other mode we find two principal groups or quartets of planets, separated by a zone or belt of a great many very small planets. { Mercury. First Group { Venus. { Earth. { Mars. The Asteroids or Planetoids. { Jupiter. Second Group { Saturn. { Uranus. { Neptune. The first four are small compared with the last four, though much larger than any in the belt of tiny Asteroids. It was believed at one time that a planet had been discovered nearer to the sun than Mercury, and the name Vulcan was given to it. But no more has been seen of Vulcan, and his existence is so doubtful that we must not count him as a member of the family without further information. Mercury is very much smaller than our Earth. The diameter of the earth is eight thousand miles, but the diameter of Mercury is only about three thousand miles--not even half that of the earth. Being so much nearer to the sun than ourselves, the pulling of his attraction is much greater, and this has to be balanced by greater speed, or Mercury would soon fall down upon the sun. Our distance from the sun is ninety-three millions of miles. Mercury’s distance is only about one-third of ours; and instead of traveling, like the earth, at the rate of eighteen miles each second, Mercury dashes headlong through space at the mad pace of twenty-nine miles each second. It is a good thing the earth does not follow his example, or she would soon break loose from the sun’s control altogether. The earth takes more than three hundred and sixty-five days, or twelve months, to journey round the sun in her orbit. That is what we call “the length of our year.” But Mercury’s year is only eighty-eight days, or not quite three of our months. No wonder!--when his pathway is so much shorter, and his speed so much greater than ours. So Mercury has four years to one year on earth; and a person who had lived on Mercury as long as five earthly years, would then be twenty years old. The increased number of birthdays would scarcely be welcome in large families, supposing we could pay a long visit there. The sun, as seen from Mercury, looks about four and a half times as large as from here; the heat and glare being increased in proportion. No moon has ever been found, belonging to Mercury.