The dawn of astronomy / A study of the temple-worship and mythology of the ancient Egyptians

by Sir Norman Lockyer

Was it difficult to do this? Did it indicate that the people who built such temples were great astronomers? Nothing of the kind; nothing is more easy to determine than a solstice or an equinox.

Let us take the solstice first. We know that at the summer solstice the sun rises and sets furthest to the north, at the winter solstice furthest to the south. We have only from any point to set up a line of stakes before the time of the solstice, and then alter the line of them day by day as the sun gets further to the north or south, until no alteration is wanted. The solstice has been found.

There is another way of doing it. Take a vertical rod. Such a rod, which I may state is sometimes called a gnomon and used to measure time, may be used with another object: we may observe the length of the shadow cast by the sun when it is lowest at the winter solstice, and when it is highest; at these two positions of the sun obviously the lengths of the shadows thrown will be different. When the noon-sun is nearest overhead in the summer the length of the shadow will be least, when the sun is most removed from the zenith the shadow will be longest.

The day on which the shortest shadow is thrown at noon will define the summer solstice; when the shadow is longest we shall have the winter solstice.

This, in fact, was the method adopted by the Chinese to determine the solstices, and from it very early they found a value of the obliquity of the ecliptic.

It may be said that this is only a statement, and that the record has been falsified; some years ago anyone who was driven by facts to come to the conclusion that any very considerable antiquity was possible in these observations met with very great difficulty. But the shortest and the longest shadows recorded (1100 years B.C.) do not really represent the true lengths at present. If anyone had forged these observations he would state such lengths as people would find to-day or to-morrow, but the lengths given were different from those which would be found to-day. Laplace, who gave considerable attention to this matter, determined what the real obliquity was at that time, and proved that the record does represent an actual observation, and not one which had been made in later years.[6]

Next suppose an ancient Egyptian wished to determine the time of an equinox. We know from the Egyptian tombs that their stock-in-trade, so far as building went, was very considerable; they had squares, they had plumb-lines, they had scales, and all that sort of thing, just as we have. He would first of all make a platform quite flat; he could do that by means of the square or plumb-line; then he would get a ruler with pretty sharp edges (and such rulers are found in their tombs), and in the morning of any day he would direct this ruler to the position of the sun when it was rising, and he would from a given point draw a line towards the sun; he would do the same thing in the evening when the sun set; he would bisect the angle made by these two lines, and it would give him naturally a north and south line, and a right angle to this would give him east and west. So that from observations of the sun on any one day in the year he would practically be in a position to determine the points at which the sun would rise and set at the equinox—that is, the true east and west points.

Suppose that the sun is rising, let a rod throw a shadow; mark the position of the shadow; at sunset we again note where the shadow falls. If the sun rises exactly in the east and sets exactly in the west, those two shadows will be continuous, and we shall have made an observation at the absolute equinox. But suppose the sun not at the equinox, a line joining the ends of the shadows equally long before and after noon will be an east and west line.

It is true that there may be a slight error unless we are very careful about the time of the year at which we make the observations, because when the sun is exactly east or west at the time of rising or setting it changes its declination most quickly. So it is better to make the above observations of the sun nearer the solstices than the equinoxes, for the reason stated.[7]

We have now got so far. If the Egyptians worshipped the sun and built temples to it, they would be more likely to choose the times of the solstices and the equinoxes than any other after its annual movement had been made out.