The Ways of the Planets

by Martha Evans Martin

VI

THE MOVEMENTS OF THE PLANETS

In considering the movements of the planets, we have to regard their actual motion in space and that motion as it appears to us. They all have two principal motions in space. They revolve about the sun in their orbits, and they rotate on their axes. The manner in which they accomplish the rotation on their axes determines the length of their days and nights, or whether, indeed, they shall have any such grateful alternations of light and darkness. Those planets which, like the earth, turn on their axes in less time than they make their journey around the sun have one day and one night every time they make a complete rotation. Those that turn on their axes in the same time that they revolve around the sun, of which sort there seems to be at least one, face always toward the sun, and have no alternations of day and night. On one side it is always day; on the other it is always night. The number of days a planet has during each revolution around the sun depends upon how much time it requires to make a revolution, and how fast it spins on its axis. In one year here on the earth we have three hundred and sixty-five days and nights. Saturn, in its year, has more than twenty-three thousand days and nights.

The manner in which the revolution of the planets in their orbits takes place determines the length and character of their year; the nearer a planet is to the sun, the shorter its orbit is, and the faster the rate of speed at which the sun compels it to move, and hence the shorter its year. The nearest of the planets, Mercury, makes more than five hundred revolutions around the sun, while the farthest, Neptune, makes one. Three times in a year—that is, a terrestrial year—the nearest planet speeds around its orbit and back to the starting-place with seventeen days to spare. One hundred and sixty-five terrestrial years are necessary for the farthest planet to make one circuit of its orbit. The first goes at the average rate of nearly thirty miles a second over a path more than two hundred million miles long. The second travels a path more than seventeen billion miles in length, at the average rate of three and four-tenths miles a second. Between these two extremes the other planets have orbits and rates of speed varying with their distances from the sun. The farther they are from the sun, the larger the orbit and the slower the speed.

To get something like a picture of the sun and the planets as they actually lie and as they move in space, one should have in mind an immense flat, circular disc five and a half billions of miles in diameter passing through the sun, which is in the center of it. Around the edge of the disc is the orbit through which Neptune moves. At varying distances inside of it are the orbits of the other planets, each growing smaller and smaller as one comes nearer and nearer to the sun, until the orbit of Mercury, the planet nearest to the sun, is reached.

Since it is not a hard metal disc that we are considering, but only an imaginary one in space, there may be a little latitude allowed for the orbits to tip somewhat out of the exact plane of the disc without materially altering the figure in mind. And this they do, very slightly—most of them to the extent only of from one to two degrees, though one of them falls outside of the common plane about seven degrees. In these orbits all the planets, as seen from the sun, are going around from west to east. At the same time they are turning on their axes in the same direction, some standing almost erect, as it were, in their orbits and whirling like a dancing dervish as they skim along, and others more or less inclined like a traveling top.

The time a planet requires to make one circuit of its orbit constitutes, as with the earth, its year. But we who are on the earth have, in our study of another planet, to regard it as having in a sense two years. First, there is the time it takes, starting from a given point in its orbit, to circle around the sun and return to that point. This is known as its sidereal period, or year, and is so called from sidus, meaning a star, because the only way to mark any point in space is by a fixed star, and, as viewed from the sun, one revolution of a planet would be from a given star back again to that star.

Then there is the time a planet takes, starting when it is in a straight line with the earth and the sun in space, to return to the place where the three bodies will be again in the same relative position. This is known as its synodic period, or year. Synodic is from our word synod, meaning a meeting or assembly, and the synodic year is the time between two successive and similar meetings of these three bodies. The sidereal year concerns the planet in its relation to the sun; the synodic year, in its relation to the earth. The synodic year is the only one that much concerns us while regarding the planets as a part of the spectacle of the sky. It is the one that we know from observation, while the sidereal year is mathematically computed.

The two periods, or years, are not of the same length, because the sun with reference to the planet is always stationary, and the motion resulting in the sidereal year is that of the planet only, while the synodic year is the result of the movements of both the earth and the planet, each, in its own orbit, being always in motion.

An inferior planet, situated as it is nearer to the sun than the earth is, and so having a shorter orbit than the earth’s, will, when it finishes its sidereal year and comes around to the point from which it started, find the earth advanced from that position and will, therefore, have to travel farther on in order to overtake it and come into the same relative position from which they started, which makes the time of its circuit with reference to the earth obviously longer than with reference to the sun.