With the superior planets the case is just reversed. The earth is the inside planet, or the one nearest the sun, and it must overtake them. With one exception, they are all so far away from the sun and move so slowly that it takes us but little more than one of our years to overtake them and bring them into the same relative position with us that they had when we started, while it requires many of our years for any one of them to make a single circuit of the sun. Hence their circuit with reference to the earth is shorter than with reference to the sun.
With Mars, the exception referred to, we have a more hardly fought race. That planet is not so far from us as are the other superior planets. It makes its revolution around the sun in a little less than two of our years. We travel eighteen miles a second, and it travels fifteen miles in the same length of time. If we are in line with it at the beginning of our journey, we glide off swiftly, and easily leave it far behind. When, however, we come back to the starting-point, it has not loitered, and is many millions of miles ahead of us, and it remains ahead until more than seven weeks after we have returned to the starting-point a second time. Fifty days after we have begun to make our third round we overtake it, and are again in a direct line with the planet and the sun. This makes its period with reference to the earth ninety-three days longer than its own year, and fifty days longer than two of ours. This is the longest synodic period among the planets.
The orbits in which the planets move all have the form of an ellipse—that is, of a circle more or less flattened. This flattening, or the extent to which an orbit departs from the form of a true circle, is called its eccentricity. The sun is never at the exact center of an orbit, but is always situated a little to one side of the center—that is, it is at one of the foci of the ellipse. Consequently, the planet, as it travels in its orbit, is not always at the same distance from the sun, the amount of the variation in distance depending upon the eccentricity of the orbit. The point in the orbit where the planet is nearest to the sun is its perihelion, and the point at which it is farthest is its aphelion. It is necessary to keep these elementary facts in mind in order fully to understand the changes in the motions and brightness of the planets.
The influence of one body over another that is circling around it is to make it move faster or more slowly according to its distance from the central body. Since a planet varies in its distance from the sun in the different parts of its orbit, it is forced to move fastest when it is in that part of the orbit which is nearest to the sun, and slowest when it is in the part farthest away. In other words, the motion of a planet is more rapid at perihelion than at aphelion. The earth is in perihelion, or nearest to the sun, in winter—that is, winter in the northern latitudes—and in consequence it moves faster in winter than in summer, and the northern winters are, for this reason, a little shorter than the summers.
These two simple movements of the planets—that around the sun and that on their axes—are their principal real movements, and are such as they would show to be if seen from the sun, which is the center of them. There are also certain minor real movements arising from various causes, one being the influence that the planets exercise on one another; but for the ordinary observer these have no particular significance. Then, the planets all share the one grand movement which the sun itself is known to be making through limitless space to a destination of which we are in utter ignorance, over even a path which we know nothing of save that it leads toward the bright star Vega, in the constellation of the Lyre. As the sun moves on in that direction at the rate of eleven miles a second he takes with him all his family of planets and planetoids, with their satellites, and whatever other bodies have their abode in his domain. Thus they travel as a body, each individual spinning on its axis, from the sun itself down to the smallest planetoid, the satellites circling around the planets, and the planets in their turn around the sun. And in all these movements the earth takes part as one of the planets. The sun itself is following a comparatively straight line in space, and, so far as we know, in allegiance to no other body. It is, though, just possible that this comparatively straight line may be the arc of a circle so vast that we have not yet had time to discover its curvature, and that the sun itself may be pursuing its own circuit around some still more powerful body.
VII
HOW THE INFERIOR PLANETS SEEM TO MOVE
Of the real movements of the planets, as described in the last chapter, we get here on the earth only a very fragmentary view. Without the aid of the telescope none of them is visible to us except the movements in their orbits, and these, to our view, are somewhat different from the simple, circling course apparent to an observer on the sun. The difference is due to the fact that the earth itself is always in movement in just the same way that the other planets are, and we, being never at any time at the center of the orbits, do not see the movements of the planets as they truly take place, but only as they are outlined against the sky. So the appearances and disappearances and visible travels among the stars by which we know the planets are only as we see them. Some knowledge of the real movements is necessary to a proper understanding of the apparent movements; but it is only with the latter that, for ordinary observation, we need to be particularly acquainted.