The motion of a planet is said to be direct when it moves in the direction of the succession of the zodiacal signs; retrograde when in the contrary direction. All of the planets have periods of retrograde and direct motion, though their usual direction is direct, from west to east. Retrograde motion can be explained by reference to the accompanying diagrams. In Fig. 4, the outer circle represents the path of the zodiac on the celestial sphere. Let the two inner circles represent the orbits of the earth and an inferior planet, Venus, around the sun, at S. (An inferior planet is one whose orbit around the sun is within that of the earth. A superior planet is one whose orbit is outside that of the earth.) V, V′ and V″, and E, E′, and E″ are successive positions of the two planets in their orbits, the arc VV″ being longer than the arc EE″ because the nearer a planet is to the sun, the greater is its velocity. Then when Venus is at V and the earth at E, we shall see Venus projected on the celestial sphere at V1. When Venus has passed on to V′ the earth will have passed to E′ and we shall see Venus on the celestial sphere at V2. The apparent motion of the planet thus far will have been direct, from west to east in the order of the signs. But when Venus is at V″ and the earth at E″ Venus will be seen at V3 having apparently moved back about two signs in a direction the reverse of that taken at first. This is called the planet’s retrograde motion. At some point beyond V″, the planet will appear to stop moving for a very short period and then resume its direct motion. In Fig. 5, the outer arc again represents the path of the zodiac on the celestial sphere. The smaller arcs represent the orbits of the superior planet, Mars, and the earth around the sun, S. At the point of opposition of Mars (when Mars and the sun are at opposite points in the heavens to an observer on the earth) we should see Mars projected on the zodiac at M1. After a month Mars will be at M′ and the earth at E′, so that in its apparent motion Mars will have retrograded to M2. After three months from opposition Mars will be at M″ and the earth at E″, making Mars appear at M3 on the celestial sphere, its motion having changed from retrograde to direct.
Fig. 5.
Both Figures 4 and 5 take no account of the fact that the earth’s orbit and those of the planets are not in exactly the same planes. Remembering this fact we see at once that the apparent oscillations of the planets are not back and forth in a straight line but in curves and spirals. It is easy to see why the apparent motions of the planets were accounted for by deferents and epicycles, before the Copernican system revealed the true nature of the solar system as heliocentric and not geocentric.
SELECTED BIBLIOGRAPHY
Berry, Arthur, A Short History of Astronomy. New York. 1899.
Bryant, W. W., A History of Astronomy. London. 1907.
Cumont, Franz, Astrology and Religion among the Greeks and Romans. New York. 1912.