Alas for permanency! The ancient Egyptians forgot in their calculations that the rising and setting points of heavenly bodies are likewise dependent upon the Earth. The stars do not rise now, where they rose in ancient times. The poles of the Earth are not remaining in one place while the Earth spins around. They too have a motion and every little movement has a meaning all its own. They are wobbling as the points of a top wobble when the top is slowing down.
It is hard to buy an old-fashioned top today; but allow us to recommend one to you, if you wish to understand Precession. Wind the top up with a cord, give it a good swift start on a flat surface; then watch it carefully. Do you notice that the tip end has a little motion of its own? Even while the spin is still so fast that the rotation is indistinguishable, the end where you wound the cord is describing a much slower circle, apparently all on its own?
In much the same way our Earth behaves. The spin, like the spin of the top, makes its day and night; but the wobble of the poles, makes precession, and this little circular movement is so slow that 25,800 years are required before the precessional hand on our grandfather’s clock will have returned once to its starting point.
Wind up the top again. (You will probably be expert at topspinning before you have understood precession, but the result is worth the effort, and anyway it makes a good excuse to play for a while.) Watch the center this time. Now do you see that while the tips are moving, the center must be wobbling too? In the same way, while the poles of our Earth are describing their little circles, the equator also moves. But the ecliptic, which is the apparent path of the Sun in the heavens, is standing still, and so the place where the Sun crosses the equator moves slowly backward, along the constellations of the zodiac; and because the motions of the tip and the center are coincident, this movement takes the same 25,800 years to complete its circuit.
The points where the path of the Sun crosses the equator are known as the Equinoxes, because as the Sun passes them, day and night upon the Earth are of equal length. The Spring Equinox was in the constellation Aries in Greek times, but it is in Pisces now, and in two hundred years or so it will pass on to Aquarius. As a result of the Precession of the Equinoxes, all the stars in the heavens seem to have moved; and because of this apparent motion the Egyptian temples no longer point to their chosen stars. By this variation the dates of their construction can be found.
Precession, then, is the fourth of the great heavenly timekeepers. The fifth, and the slowest worth mentioning here, is the wheellike rotation of the Milky Way. It is so slow that a hand on the grandfather’s clock would take something like 530,000,000 years to complete a circuit. For practical purposes, such a hand would have little use; but if, and when, interstellar radio becomes an accomplished fact, we will need some such device to time our messages. Then clocks will have to be built more carefully than they are today; and a permanent notation must be chiseled deep in stone, so that our descendants of some billion years from now, will remember to wind the thing up.
With all these varied heavenly motions to confuse us, it is hardly surprising that astronomers found difficulty in differentiating the kinds of time. The arguments which arose were like the discussions between two men whose watches disagreed; and astronomers who are responsible for their setting have no town-hall clock or radio to consult for ultimate authority.
The man who has set his watch by the stars, calls his kind of time “sidereal,” and he swears by it. The man who has set his watch by the apparent movement of the Sun can prove that “apparent solar time” is of practical value; and the man with his watch set to the average of the Sun’s time throughout the year, can point to all the people on the street, and clinch his point: “They are using ‘mean solar time’ too.”
All these various kinds of time are defined in the glossary, but the last speaker was perfectly correct. Since we use today only mean solar time for our daily unastronomical lives, we shall not deal with the others here.
“Mean Solar Time” to which we, and the town-hall clock, and the radio time signals, all refer, is the average time kept by the Sun throughout all the days of the year. The actual solar day may be half a minute more or less than the mean solar day, and the accumulations of this difference may amount to as much as fifteen minutes. Therefore our sundials and our watches usually disagree. Before mechanized clocks were invented men counted hours by the shadow of the Sun on the dial. But our passion for standardization has changed even the nature of hours themselves; today we average up all the days of the year, and take the mean, setting both our clocks and our lives accordingly.
Yet even within the province of routine existence, where mean solar time holds complete sway, there have been some differences of opinion. Should we, for example, reckon the beginning of the day from the average midnight? or from the average high noon? or even from the average sunset? From midnight, of course. We have a hard enough time remembering the correct date without changing our calendar when we come back from lunch. Yet old tradition reckons always from sunset to sunset. A dark line is always used to mark divisions both of space and time. In almost all calendars which reckoned months by our satellite the dark of the Moon was the turning point. And in the Bible “the evening and the morning were the second day.” Hebrew ritual keeps this division still, and begins its Sabbath at the sunset hour; but even in Babylonian times the merchants and the lawyers, the tax collectors and the artisans found midnight more convenient.
Disagreements arose. In those happy and far-off days, there was another class of people who were as important as the merchants and as powerful as the priests. The opinions of the astronomers were consulted almost as much as the laws of the kings, and astronomers, like owls, see more clearly in the dark. For them it was better if the day changed at high noon. They could not be bothered to turn over a page of their calendars, metaphorically speaking, at the moment when they were in the middle of their most complicated observations. By the dictates of these noble personages, the astronomical day began at noon, although the businessman changed his at night. Confusion ensued in Babylon-a confusion which was to last for three thousand years. By A. D.