The motions of the moon, eclipses, and astrology
The full moon is shown at the right as you see it with your unaided vision or binoculars. Most lunar maps are plotted as a telescopic view, inverted and reversed, so it is difficult to correlate them with a normal view. The labelled points of interest are: the craters Plato (P), Eratosthenes (E), Copernicus (C), Kepler (K), Gassendi (G), and Tycho (T), from which the rays are prominent; the maria Crisium (c), Serenitatis (s), Tranquillitatis (t), Fecunditatis (f), Nectaris (n), Nubium (n'), Imbrium (i), Humorum (h); the Oceanus Procellarum (p); and the Lunar Apennines (α). These landmarks will orient you. The craters are the result of impacts over billions of years; the maria are flows of basaltic lava that have covered the earlier surface, but are still extremely old. No change takes place on the surface of the moon. We see only one side of the moon because its rotation has been captured by the earth, due to tidal forces, so its rotational period is equal to its orbital period. The other side of the moon is similar, but lacks the large maria of the side facing us. Galileo was the first to realize that the moon's surface is rough and cratered, when he observed it in his new telescope in 1610. Previously, it had been assumed to be smooth, cloudy crystal. The moon is obviously quite spherical in any case.
The moon's orbit about the earth is an ellipse of eccentricity 0.0549 inclined by 5° with the ecliptic, the plane of the earth's orbit. The points where the planes of the orbits cross are the nodes. These nodes move westward along the ecliptic rather rapidly, taking 18.6 years to go all the way round. The point where the moon is nearest earth, the perigee, moves eastward, taking 8.8 years for a complete circle. The movement of the moon against the stars is, therefore, quite complicated and variable. Nevertheless, the moon remains within the zodiacal band along the ecliptic.
Since the full moon is opposite the sun on the celestial sphere, when the sun is high, as in the summer, the full moon is low, while when the sun is low, as in the winter, the full moon is high in the sky. One can tell the season by observing the full moon in the sky.
The moon moves eastward in its orbit with respect to the stars, completing one circuit in 27.3 days. The sun also moves eastward, so it returns to the sun every 29.5 days, which is the period of the phases of the moon. Since the moon's orbit is inclined, the sun and moon usually miss each other as seen from the earth. However, if the moon is near a node of its orbit when it passes the sun, there is a chance of a solar eclipse. If it is near a node when the sun is 180° away, there is a chance of a lunar eclipse. Eclipses only occur at new and full moons. Since the sun and moon are about the same apparent size, half a degree, the coincidence must be very good for a total eclipse of the sun. The earth's shadow is larger than the moon, so lunar eclipses are much more frequent. Early Greek astronomers noticed that this shadow was circular, and gave this as evidence for the sphericity of the earth. In all other astronomies, the earth was flat, even if the celestial vault was a sphere.
The moon appears to move about 13° eastward every day relative to the stars. This means it moves about its own diameter in an hour, a movement that you can easily see with careful watching. The moon is, in fact, a clock as well as a calendar. To see this, however, you need a fixed reference point, such as a bright nearby star. Both star and moon will be carried rapidly westward, 15° an hour, but the moon will also move a little eastward with respect to the star as you watch for an hour or so.
The moon was the first calendar. Its motions could be easily followed by night, its changes of phase seen by all. In order to make this quantitative, standard reference points were necessary, and these were provided by the stars. Groups of stars were picked out along the moon's path that marked the daily motion eastward. Usually, 28 such references were established, and they were called the Lunar Mansions, or resting-places. The position of the moon could be specified relative to them, and the complex motion of the moon clearly revealed. Lunar Mansions were prominent in the astrology of the East, in China, India, Arabia, Egypt, Persia, central Asia, and, above all, in the Euphrates valley among the Chaldeans, where they may have originated.
We now speak glibly of the postion of the sun among the stars, and do not think how it could have been obtained in the absence of accurate timepieces. We can now observe the sun by day with a transit instrument, and the clock will tell us its right ascension. Ancient astronomers had a more difficult time, using water clocks to measure the interval between the setting of the sun and the setting of a bright reference star. It is much more difficult to determine the position of the sun than that of the moon. The Lunar Mansions probably antedated the Zodiac, for this very reason. If the path of the ecliptic is known, the altitude of the sun, as obtained by a gnomon, also gives its position. It was noted by Hipparchus that the motion of the sun was not uniform, but a little faster in winter and a little slower in summer.
In order to predict a solar eclipse, it is necessary to know the position of the moon at the instant when the sun occupies a node of the moon's orbit. The prediction of a total, annular or partial eclipse also depends on knowing the moon's apparent size, which is larger at perigee, smaller at apogee. A simple prediction weeks or months in advance was at the very limit of classical Greek astronomy, and was impossible in the astronomies of other cultures. There was no difficulty in predicting the chance of an eclipse, and a short-term prediction was much easier since the relative positions of sun and moon could be observed. Possibly, it was better for an astronomer to report the chance of an eclipse and have it not happen, than to stay silent when an eclipse occurred, leaving the Emperor unwarned, as apparently happened in China. The astronomer could always say that his diligent actions warded off the disaster of an eclipse, after all. This seems to resemble the practices of modern TV weather forecasters.
The gravity of the negligent astronomer's crime arose from the belief that the stars foretold the future. Any irregular event was a portent of catastrophe. In every astronomy except the Greek, the sole reason for the study of the stars was for this purpose, and even among Greeks the predictive power of the stars was generally believed. The rather superstitious Greeks, in fact, took the legacy of Chaldean star lore and elaborated it into the astrology that is still practiced, as in Aratos' Phaenomena of 270 BC. From this beginning, it became the most successful fraud ever known. Its quasi-scientific nature appealed to people who were aware of the improbability of gods.
We now call the study of the supernatural influence of stars and planets astrology, and the scientific study astronomy, but the words have not always had these meanings. The Roman government expelled the astrologers from Rome in 139 BC, after the merging of Roman and Greek culture had allowed hordes of these practitioners in. They called themselves, variously, Babyloni, Chaldaei, Astronomi, Astrologi, Genethliaci, Mathematici and Planetarii, and all claimed to be able to determine the future from the positions of the planets, while offering other useful services. Seneca was the first writer whose works still exist who explicitly distinguished between astronomy for knowledge and astronomy for prediction. His scepticism was as widespread among the educated as a belief in the powers of the stars was (and still is) among the general public.
Today, Lunar Mansions may be mentioned by astrologers who have little appreciation for what they actually signify, and cling to such ancient practices because they sound mysterious and esoteric. Men have actually set foot on the moon, and we know that the celestial objects are just inanimate masses of rock and gas, not spirits. Truth can hardly quench the thirst for knowledge that cannot be had, however, so a belief in spirits continues, however ineffective and silent the spirits are. The same incentive drives Lottery sales, but here there is a much better chance of winning.
Composed by J. B. Calvert
Created 5 October 2000
Last revised 2 August 2001