The first great division of time is marked by day and night
 |
The first great division of time is marked by day and night. The Earth rotates on its axis like an orange strung on a wire before a steady light and the glow falls first on one side then on the other. Were we situated on a distant star, we should see in this motion the most perfect regularity known to nature. The period is so even that astronomers are doubtful whether there has been any change in its pace since the beginning of historical time. From our prejudiced viewpoint on the surface of the rotating planet, this perfect regularity seems upset by another, quite different motion--the motion which causes the seasons--the revolution of the Earth around the Sun. There is nothing even about this second movement, and its unevenness seems, to our prejudiced eyes, to upset the regularity of day and night. Consequently the standard observatories make their readings from the time when one star crosses the meridian to the time when the same star recrosses, rather than by the crossings of the Sun. By this means they manage to pretend that they are watching our planet from the distant star itself, and their viewpoint becomes impartial. Yet the reckonings, whether made by an amateur taking time by the Sun or a professional gauging it by a distant star, are both made to agree with the Earth's motion on its own axis; and by that the twenty-four hours are set.
Equally important for us is the revolution of the Earth around the Sun, and on this revolution our calendars are based. All our "anniversaries" (from their very name), our festivals, the seasons which mark the planting and reaping of crops, and, in specific cases, the overflowing of rivers, are dependent upon this revolution. The period of the year is very nearly constant, but the half-year periods are variable. In our time the Sun prefers its northern residence, and lingers there three and a half days overtime, so that summer in the northern hemisphere is longer than summer in the antipodes.
These three celestial clocks, the rotation of the Earth on its axis, the revolution of the Earth around the Sun, and the revolution of the Moon around the Earth, have formed the bases of time in our practical, everyday lives.
There are, however, a number of other celestial timekeepers. The periods of time, which they mark, are so long that the motions can hardly be observed in the course of a normal life; but historically they are of vast importance.
The most interesting of these is the Precession of the Equinoxes, which deserves and receives a full chapter to itself, later in the book. Had we lived in the days when time was measured by the slow pendulum swing of a grandfather's clock, with one hand measuring the hours, and another moving day by day to reckon the year, we might have been tempted to construct a third hand, longer than the others; and around the edge of the dial we should have placed the signs of the zodiac. This hand would take 25,800 years to complete a single circuit. In our ordinary lives of three score years and ten, the hand would move one degree of arc. That seems very slow, and not very consequential. Its very great slowness may be the reason why even our grandfathers, who moved so much more leisurely than we, never thought to mark it on their clocks. But as a matter of fact the movement has been marked. We can determine it, and it has helped modern historians in the dating of ancient temples.
The dating was, of course, purely accidental. If the ancient Egyptians had had the faintest idea of this slow change, they might never have marked it for us at all. What they wanted most in the world was permanency. Therefore they built some of their temples to face the most permanent things they knew--the rising and setting points of stars. Everything on the surface of the world changes. But surely, they thought, the stars will always set at the same place. No one can gauge by mountains: weather can change them, so can Mohammed and faith, if legend hold true. Still less can anyone gauge by the shadow of a tree or a stone. But the heavenly bodies are above the Earth. "So," they must have said, "by the rising or setting of stars, our temples will be fixed. By them we can reckon the year, and our sons and our sons' sons can reckon it as we do. We have hitched our temples to their stars for all time."
|
||
