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CHAPTER II

THE ASTRONOMICAL DIVISIONS OF THE YEAR

IT is next important to deal with the yearly path of the sun, with a view of studying the relation of the various points of the horizon occupied by the sun at different times in the year. In the very early observations that were made in Egypt, Chaldæa and elsewhere, when the sun was considered to be a god who every morning got into his boat and floated across space, there was no particular reason for considering the amplitude at which the boat left, or came to, shore. But a few centuries showed that this rising or setting of the sun in widely varying amplitudes at different times of the year at the same place obeyed a very definite law.

In its northward passage it reaches the highest point at our summer solstice, and then goes down again till it reaches its greatest southern declination, as it does in our winter. At both these points the sun appears to stand still in its north or south movement, and the Latin word solstice exactly expresses that idea. The change of declination brought about by these movements will affect the place of the sun's rising and setting; this is why the sun sets most to the north in summer and most to the south in winter. At the equinoxes the sun has always 0° Decl., so it rises and sets due east and west all over the world. But at the solstices it has its greatest declination of 23½° N. or S.; it will rise and set therefore furthest from the east and west points; how far, will depend upon the latitude of the place, as will have been gathered from the preceding table (p. 11).

These solstices and their accompaniments are among the striking things in the natural world. In the winter solstice we have the depth of winter, in the summer solstice we have the height of summer, while at the equinoxes we have but transitional changes; in other words, while the solstices point out for us the conditions of greatest heat and greatest cold, the equinoxes point out for us those two times of the year at which the temperature conditions are very nearly equal, although of course in the one case we are saying good-bye to summer and in the other to winter.

Did the ancients know anything about these solstices and these equinoxes? Dealing with the monumental evidence in Egypt alone, the answer is absolutely overwhelming. Many thousand years ago the Egyptians were perfectly familiar with the solstices, and therefore with the yearly path of the sun.

This fundamental division of the sun's apparent revolution and course which define our year into four nearly equal parts may be indicated as in Fig. 5, the highest point reached by the sun in our northern hemisphere being represented at the top.

FIG. 5.—The four Astronomical Divisions of the year.

In order better to consider the problem as it was presented to the early astronomers who built observatories (temples) to mark these points, we may deal with the bearings of the points occupied by the sun on the horizon (either at rising or setting) at the times indicated. These points are defined, as we have seen, by their "amplitude" or their distance in degrees from the E. or W. points of the horizon. In the diagram (Fig. 6) I represent the conditions of our chief British sun-temple, Stonehenge, in latitude 51° N. approximately.

FIG. 6.—The various bearings of the s in risings and settings in a place with a N. latitude of 51°.

Taking the astronomical facts regarding the solstices. and equinoxes for the first year (1901) of the present century, we find—

Sun enters Aries, Spring equinox, March 21.
Sun enters Gemini, Summer solstice, June 21.
Sun enters Libra, Autumn equinox, September 23.
Sun enters Sagittarius, Winter solstice, December 23.


These points, then, are approximately ninety-one days apart (91 × 4 = 364).

In Fig. 6 I deal with the "amplitudes" at Stonehenge, that is, the angular distance along the horizon from the E. and W. points, at which the sunrise and sunset are seen at the solstices; at the equinoxes they are seen at the E. and W. points. But as these amplitudes vary with the latitude and therefore depend upon the place of observation, a more general treatment is possible if we deal with the declination of the sun itself, that is, its angular distance from the equator.

The maximum declination depends upon the obliquity of the ecliptic, that is, the angle between the plane of the ecliptic and that of the equator at the time of observation. When the Stonehenge Sarsen Stones were erected this angle was, as I shall show later on, 23° 54´ 30?. Its mean value for the present year (1906) is 23° 27´ 5?; it is decreasing very slowly.

It will be obvious from Fig. 6 that in temples built to observe the solstices or equinoxes, if they were open from end to end, looking in one direction we should see the sun rising at a solstice or equinox, and looking in the other we should see the sun setting at the opposite one. I shall show later on that this statement requires a slight modification.

But temples so built interfered with the ceremonial, which required that the light should illuminate a naos—that is, the Sanctuary or Holy of Holies, only entered by the High Priest, and generally kept dark. Usually, therefore, two temples were built back to back, with a common axis, as at Karnak.

And here a very important point comes in; which time of the year and day of the year are most easy to fix by astronomical observation? As a matter of fact the summer solstice, the position of the sun on the longest day, is a point easily fixed. All we have to do is to observe the sun rising more and more to the north as the summer approaches, until at the very height of the summer we have the extreme north-easterly point of the horizon reached, and the sun stands still. We have the solstice. We can then put a row of stakes up, and so fix the solstitial line. Of course we find, as mankind has found generally, that the sun comes back next year to that same solstitial place of rising or setting. So that when we have once got such an alignment for the rising of the sun at midsummer, we can determine the length of the year in days, and therefore the beginning of each year as it comes round.

So much, then, for the chief points in what we may term the astronomical year, those at which the sun's declination is greatest and least. We see that they are approximately ninety-one days apart—say three months.
 

Next Chapter: Chapter III. The Agricultural Divisions of the Year