In the priestly calendar lore, magic and genuine science were inextricably entangled. The social necessity of measuring time arises from the seasonal fertility of man's biological allies, and the earliest explanations of the celestial events were frequently mixed up with man's preoccupation concerning his own fertility. What are sometimes offered as rival explanations of early practices are really different ways of saying the same thing. The phallic tension of waking and the monthly cycle of a woman's life were closely associated with sunrise and lunar phenomena in the thought of primitive man. To say that an obelisk is a sundial, and to say that it is a phallic symbol involves no contradiction. Fertility and timekeeping were very closely connected in the same social context. Man had to be disciplined into the recognition that his own world is not the centre of the astronomical universe. He had to outgrow the belief that his own person is a sufficient model of natural processes in chemistry and biology. In psychology and social science he has still to learn that individual preference is not a safe guide to the understanding of social behaviour.
As liaison officers to the celestial beings, the priests found it paid to encourage the belief that nature can be bought off with bribes like a big chief. One of their most powerful weapons was their ability to forecast eclipses. Eclipses were indisputable signs of divine disapproval, and divine disapproval provided a cogent justification for raising the divine income tax. No practical utility other than the advancement of the priestly prestige and the wealth of the priesthood can account for the astonishingly painstaking attention paid to these phenomena. The moon's track lies very close to the ecliptic. If it moved exactly in the plane of the ecliptic, there would be a central eclipse of the sun every new moon, and a total eclipse of the moon every month, at the full. Careful measurement shows that its orbit is inclined about 5° to the ecliptic (i.e. to the plane of the earth's orbit, as we now say). So the moon's path round the earth only cuts the earth's path round the sun (or the sun's apparent track around the earth) at two points called nodes, and an eclipse can only take place if the moon is at, or very near to, a node when the two nodes are in line with the sun and the earth. Relative to the fixed stars, the direction of the line which joins the nodes rotates slowly. The sun passes a particular node every 346·62 days. This is less than a year because the nodes are moving from east to west, and meet the sun before it completes its yearly circuit. So if earth, moon, nodes, and sun are in line at any time, they will be in line once more about eighteen years * later. More precisely this period is 18 years 11 1/3 days. If an eclipse occurs on a particular date somewhere on the earth's surface, another one will occur 18 years and 11 1/3 days later at a place about 120° W. on account of the odd third of a day. This cycle is still called the Saros, which is the name given to it by the Chaldean priests. It did not help people to arrange their meal-times and night journeys, to prepare for the lambing season, or to sow their crops. For the art of time reckoning the Saros had no particular use. Its discovery was prompted by a combination of superstition and racketeering. Once made, it served to direct attention to two of the basic principles of scientific geography. Observation of eclipses in different places showed that solar time is local; and confirmed the belief that the earth is a spherical object. The fact that lunar eclipses occur when the moon is practically in the ecliptic plane shows that the circular edge of the shadow on its face is the shadow of the earth itself.