The mystery of time encloses all things in its folds, and our grasp of its infinite bearings is measured by our limitations. As there are no isolated facts in the Universe, we can never get to the end of our subject; so we know only what we have capacity to absorb. In considering the foundation on which all our time measuring is based, we are led into the fringe of that Elysian field of science—astronomy. A science more poetical than poetry—more charming than the optimistic phantasies of youth. That science which leaves our imagination helpless; for its facts are more wonderful than our extremest mental flights. The science of vastness and interminable distances which our puny figures fail to express. “The stars sang together for joy,” might almost be placed in the category of facts; while the music of the spheres may now be considered a mathematical reality. Our time keeping is inevitably associated with these motions, and we must select one which has periods not too long. That is, no continuous motion could be used, unless it passed some species of milestones which we could observe. Consequently, our clocks do not—in the strict sense—measure time; but are adjusted to divide periods which they do not determine. We are constantly correcting their errors and never entirely succeed in getting them to run accurately to periods of time which exist entirely outside of such little things as men and clocks. So a clock is better as it approximates or bears a regular relation to some motion in nature. The sidereal clock of the astronomer does run to a regular motion; but our 24-hour clocks do not, as we shall see later. Now consider the year, or the sun's apparent motion in the Zodiac, from any given star around to the same one again. This is altogether too long to be divided by clocks, as we cannot make a clock which could be depended on for anywhere near a year. The next shorter period is that of a “moon.” This is also a little too long, is not easily observed, and requires all sorts of corrections. Observations of the moon at sea are so difficult and subject to error that mariners use them only as a last resort. If a little freedom of language is permissible, I would say that the moon has a bad character all around, largely on account of her long association with superstition, false theology and heathen feasts. She has not purged herself even to this day! The ancients were probably right when they called erratic and ill-balanced persons “luny.” Now we come to the day and find that it is about the right practical length—but what kind of a day? As there are five kinds we ought to be able to select one good enough. They are:—
- 1st. The solar day, or noon to noon by the sun.
- 2nd. An imaginary sun moving uniformly in the ecliptic.
- 3rd. A second imaginary sun moving uniformly parallel to the equator at all seasons of the year.
- 4th. One absolute rotation of the earth.
- 5th. One rotation of the earth measured from the node, or point, of the spring equinox.
The difference between 1st and 2nd is that part of the sun's error due to the elliptical orbit of the earth.
The other part of the sun's error—and the larger—between 2nd and 3rd is that due to the obliquity of the ecliptic to the equator.
The whole error between 1st and 3rd is the “equation of time” as shown for even minutes in the first chapter under the heading, “Sun on Noon Mark 1909.”
Stated simply, for our present purpose, 1st is sundial time, and 3rd our 24-hour clock time.
This 2nd day is therefore a refinement of the astronomers to separate the two principal causes of the sun's error, and I think we ought to handle it cautiously, or my friend, Professor Todd, might rap us over the knuckles for being presumptuous.
This 5th day is the sidereal day of the astronomers and is the basis of our time, so it is entitled to a little attention. I shall confine “sidereal day” to this 5th to avoid confusion with 4th. If you will extend the plane of the equator into the star sphere, you have the celestial equator. When the center of the sun passes through this plane on his journey north, in the Spring, we say, “the sun has crossed the line.” This is a distant point in the Zodiac which can be determined for any given year by reference to the fixed stars. To avoid technicalities as much as possible we will call it the point of the Spring equinox. This is really the point which determines the common year, or year of the seasons. Using popular language, the seasons are marked by four points,—Spring equinox—longest day—; Autumnal equinox—shortest day. This would be very simple if the equinoctial points would stay in the same places in the star sphere; but we find that they creep westward each year to the extent of 50 seconds of arc in the great celestial circle of the Zodiac. This is called the precession of the equinoxes. The year is measured from Spring equinox to Spring equinox again; but each year it comes 50 seconds of arc less than a full revolution of the earth around the sun. Therefore if we measured our year by a full revolution we would displace the months with reference to the seasons till the hot weather would come in January and the cold weather in July in about 13,000 years; or a complete revolution of the seasons back to where we are, in 26,000 years. Leaving out fractions to make the illustration plain, we have:—
| (1) | 360 degrees of Zodiac | = 26,000 years | |
| 50 seconds of arc | |||
| (2) | 1 day of time | = 26,000 years | |
| 31⁄3 seconds | |||
| (3) | 1 year of time | = 26,000 years | |
| 201⁄3 minutes | |||
| (4) | 31⁄3 seconds | = 1⁄110 of a second | |
| days in a year | |||
| all Approximate | |||
In (1) we see that a “precession” of 50 seconds of arc will bring the Spring equinox around in 26,000 years.