| Mercury | revolves | around | the sun | in nearly | 3 | months. | |
| Venus, | " | " | " | " | 7½ | " | |
| Earth, | " | " | " | " | 1 | year. | |
| Mars, | " | " | " | " | 2 | years. | |
| Ceres, | " | " | " | " | 4 2⁄3; | " | |
| Jupiter, | " | " | " | " | 12 | " | |
| Saturn, | " | " | " | " | 29 | " | |
| Uranus, | " | " | " | " | 84 | " |
From this view, it appears that the planets nearest the sun move most rapidly. Thus, Mercury performs nearly three hundred and fifty revolutions while Uranus performs one. The apparent progress of the most distant planets around the sun is exceedingly slow. Uranus advances only a little more than four degrees in a whole year; so that we find this planet occupying the same sign, and of course remaining nearly in the same part of the heavens, for several years in succession.
After this comparative view of the planets in general, let us now look at them individually; and first, of the inferior planets, Mercury and Venus.
Mercury and Venus, having their orbits so far within that of the earth, appear to us as attendants upon the sun. Mercury never appears further from the sun than twenty-nine degrees, and seldom so far; and Venus, never more than about forty-seven degrees. Both planets, therefore, appear either in the west soon after sunset, or in the east a little before sunrise. In high latitudes, where the twilight is long, Mercury can seldom be seen with the naked eye, and then only when its angular distance from the sun is greatest. Copernicus, the great Prussian astronomer, (who first distinctly established the order of the solar system, as at present received,) lamented, on his death-bed, that he had never been able to obtain a sight of Mercury; and Delambre, a distinguished astronomer of France, saw it but twice. In our latitude, however, we may see this planet for several evenings and mornings, if we will watch the time (as usually given in the almanac) when it is at its greatest elongations from the sun. It will not, however, remain long for our gaze, but will soon run back to the sun. The reason of this will be readily understood from the following diagram, Fig. 50. Let S represent the sun, E, the earth, and M, N, Mercury at its greatest elongations from the sun, and O Z P, a portion of the sky. Then, since we refer all distant bodies to the same concave sphere of the heavens, it is evident that we should see the sun at Z, and Mercury at O, when at its greatest eastern elongation, and at P, when at its greatest western elongation; and while passing from M to N through Q, it would appear to describe the arc O P; and while passing from N to M through R, it would appear to run back across the sun on the same arc. It is further evident that it would be visible only when at or near one of its greatest elongations; being at all other times so near the sun as to be lost in his light.
Fig. 50.
A planet is said to be in conjunction with the sun when it is seen in the same part of the heavens with the sun. Mercury and Venus have each two conjunctions, the inferior and the superior conjunction. The inferior conjunction is its position when in conjunction on the same side of the sun with the earth, as at Q, in the figure; the superior conjunction is its position when on the side of the sun most distant from the earth, as at R.
The time which a planet occupies in making one entire circuit of the heavens, from any star, until it comes round to the same star again, is called its sidereal revolution. The period occupied by a planet between two successive conjunctions with the earth is called its synodical revolution. Both the planet and the earth being in motion, the time of the synodical revolution of Mercury or Venus exceeds that of the sidereal; for when the planet comes round to the place where it before overtook the earth, it does not find the earth at that point, but far in advance of it. Thus, let Mercury come into inferior conjunction with the earth at C, Fig. 51. In about eighty-eight days, the planet will come round to the same point again; but, mean-while, the earth has moved forward through the arc E E´, and will continue to move while the planet is moving more rapidly to overtake her; the case being analogous to that of the hour and minute hand of a clock.
Fig. 51.