HOW TO CALCULATE THE PLANETS’ POSITIONS 73
it completes a revolution around the Sun in 88 days. Uranus travels only 15000 miles an hour, but its circle is so large that it requires 84 years to complete. The other planets show similar variations of speed. If they traveled in a straight line the smaller and faster planets would soon leave the more ponderous and slow-moving behind, but as they move in circles, they pass a given point of observation again and again. Were that point central and stationary this constant forward motion of the planets in their respective orbits would be apparent to all observers; but that is the trouble, there is no stationary point; every particle, from Jupiter, the giant of our solar system, to the smallest particle of “stardust” is in incessant motion around a common center, and therefore, at times one planet moves almost transversely to the path of another moving body and it appears for a time as if it stood still in its orbit. Astronomers say that such a planet is Stationary. At other times this oblique motion of the planets relative to the earth’s position in its orbit makes them seem to move backward in the zodiac, and this we call Retrograde. In the ephemeris we find a capital R in line with the day when any planet commences thus seemingly to recede, and this retrogradation goes on until we find a capital D which indicates that a direct forward motion of the planet is again observable.
Though this backward motion of a planet is only seeming, it has a very real effect in respect to the in-
