NEPTUNE, the very farthest off world of all known to us, journeys round the Sun at a distance of about 2,800 millions of miles, or _eighty times as far off as Mercury_. It is not very easy to see in our minds what this means. We must climb up to the thought, step by step. Think first of a rope one hundred miles long. Perhaps you have gone in the train from New York to Philadelphia. A rope one hundred miles in length would reach all the way and ten miles farther. Next, think of ten such ropes joined together, making a single rope one thousand miles long. Then think of twenty-five of those ropes joined into one rope, 25,000 miles long. This rope would just about go round the Earth, lying on the equator like a girdle. It would take _ten_ such Earth-girdles to reach straight from the Earth to the Moon. But we have to get the thought of one million miles. Well, you would need about _forty_ Earth-girdles--forty ropes, each one being 25,000 miles long--to make a rope one million miles in length. And when we get so far it is still only one million. Mercury is thirty-five millions of miles away from the Sun. So, for the distance of Mercury, you would need--first, forty Earth-girdles joined into a one-million mile rope, and then thirty-five of those million-mile ropes, to stretch all the great way from the Sun to his nearest planet, Mercury. When you have in mind that enormously long rope, reaching from the Sun to Mercury, the rest is easier. _Two_ such ropes would about reach from the Sun to Venus. _Three_ such ropes would about reach from the Sun to our Earth. _Four_ such ropes would about reach from the Sun to Mars. But to reach from the Sun to Jupiter no less than _fourteen_ such ropes would be needed. And to reach all the way to the distant Neptune, from the Sun, _eighty_ such ropes would be needed! There indeed we find ourselves in a region of dimness and fearful cold. We can hardly fancy any human beings like ourselves living at so enormous a distance from the storehouse of light and heat. Our bright and glorious Sun, seen from Neptune, looks no larger than the planet Venus looks to us here. You and I on Earth have _nine hundred times_ as much light, and _nine hundred times_ as much heat, from the Sun, as a man on Neptune would have. Of course, if Neptune is only partly cooled, there may be plenty of heat from the planet itself. However, you must not think that the Sun even there looks only like Venus or Jupiter in our sky. Though small in size, he shines dazzlingly still. But after what we enjoy on Earth Neptune would indeed to us be a world of darkness. Travelling at the rate of three miles in a second, Neptune gets once round the Sun in 165 of our years. This planet was not discovered by accident, but through careful searching. Some day you will read with interest the story of how and why it was hunted for in the sky--and found.[2] [2] See “Sun, Moon and Stars,” pp. 227-234.