The Ways of the Planets

by Martha Evans Martin

VIII

HOW THE SUPERIOR PLANETS SEEM TO MOVE

The movements of the superior planets, Mars, Jupiter, Saturn, Uranus, and Neptune, as they appear to us, are different from those of the inferior planets in some important respects. Instead of swinging back and forth east and west of the sun, and never appearing very far away from it, as the inferior planets do, the superior planets make an entire circuit of the heavens, and it is possible to see them at any distance from the sun, and at any time during the night. Sometimes they are, with relation to the earth, in that part of the sky exactly opposite to the sun, and hence in line with it and the earth. At such times they can be seen all night. They are then said to be in opposition, and are in the best position for our observation. The earth being, when in this situation, in a direct line between them and the sun, we have the sun at our backs, as it were, shedding its full rays on the disc of the planet under observation, which is then at its nearest to us, and also at its brightest. For, since the orbits of all the superior planets are outside of ours, the planets never get between us and the sun, and, in consequence, never turn a dark side toward us. Their entire discs are practically always illuminated, and their changes in brightness depend largely upon their changes in distance, which, as we have seen, is not the case with the inferior planets.

Mars, the nearest of them, is at times somewhat gibbous (that is, shows a little less than a full face, as the moon does when just beginning to wane), and, in less degree, Jupiter also. But in neither case is this departure from fullness sufficient to have any appreciable effect on the planet’s brightness, and, moreover, it does not occur when the planet is in the most favorable position for us to see it. At opposition, therefore, we always have the full face of the planet presented to us; and being, as we then are, on the same side of the sun with it, we are ninety-three millions of miles (our distance from the sun) nearer to it than the sun is.

Being, when in opposition, exactly opposite the sun, the planet rises just as the sun sets. After opposition it rises a little earlier each evening, and is higher up in the sky at each succeeding sunset. When we find it just half-way between the eastern and the western horizon at sunset, it is at quadrature. After quadrature it appears nearer and nearer the western horizon each evening at sunset, until it finally is too near the sun to be visible. It is then traveling in that part of its orbit which is beyond the sun from us. From opposition to this situation it has been an evening star.

When a superior planet is in line with the sun and the earth, and is on the far side of the sun from us, it is said to be in conjunction, and we are then one hundred and eighty-six millions of miles, or twice our distance from the sun, farther from it than we are when it is in opposition. But besides being placed at so much greater distance from it, we have in this situation the bright sun excluding the planet from our view. It will be readily seen, therefore, why the superior planets are in so much better position for us to see them in opposition than at conjunction.

From conjunction to opposition the planet is west of the sun, and will be below the horizon at sunset, and will rise some time during the night. At first it will appear just before sunrise as a morning star, but will gradually rise earlier each night until, when it reaches opposition again, it will rise just as the sun sets. Half-way between conjunction and opposition it is again at quadrature.

From opposition to conjunction the planet will be east of the sun and above the horizon at sunset. When a planet is in conjunction with the sun, it passes the meridian, or the point half-way between rising and setting, about noon, and is above the horizon with the sun during the day. When it is in opposition it passes the meridian about midnight, and is above the horizon during the night. When it is at quadrature and moving toward conjunction, it passes the meridian about six o’clock in the evening, and may be seen in the western half of the sky during the early evening, and will set before midnight. When it is at quadrature and moving toward opposition, it will rise some time between midnight and sunset, and will be in view in the east during a part of the first half of the night. The nearer it is to opposition, the earlier in the evening it rises and the longer it may be seen.

The main movement of the superior planets among the stars is from west to east, and this is known as their direct motion. But not far from opposition they seem to hesitate, then move more slowly, then finally stop, remain stationary for a time, turn back on their tracks, and start off in the opposite direction. This is their retrograde motion. They do not continue in it as long as in the direct motion; but after a comparatively short time they again hesitate, go more slowly, stop, remain stationary, then turn back and swing off in the original direction, and continue to move in this direction until they are again approaching opposition. It is exactly in the middle of this sweep toward the west that the planet is in opposition. Close observation will show that the superior planets also make something of the same sort of a loop in their path among the stars that the inferior planets make, and for the same reason. The only difference is that when a superior planet is retrograding we are passing it, and when an inferior planet retrogrades it is passing us.