Page:Popular Science Monthly Volume 29.djvu/536

520 however, the recognized day in most countries till comparatively recent times. In France, when in 1816 the change was made to our present system, there were fears of a disturbance among working-people, lest the abolition of the sun-day should somehow increase their hours of labor. It met the approval of the watch-makers, however, whose customers had hitherto complained that their watches would not keep pace with the sun, not knowing that this would be impossible for a good watch.

The time of the rotation of the earth on its axis can not be measured directly from the sun, for the reason that the earth is moving around it. We must have some external point, fixed with reference to the earth, by which to measure it. The stars afford such points. By noticing the time between two successive passages of a star over our meridian (our meridian being, as is well known, the semicircle in the sky passing from the north to the south point of the horizon directly overhead), we would obtain the exact time of the earth's completing one spin on its axis. This time, which is about four minutes less than our ordinary day, is called in astronomical parlance a sidereal day, and, divided in the ordinary manner into hours, minutes, and seconds, is known as sidereal time. It has no direct relations to ordinary life.

Through all the time that the earth is making one turn on its axis it is advancing around the sun in the same direction. So it takes this extra four minutes to bring the same meridian under the sun again, after making a complete revolution. Hence we have our solar day. Again, since the forward motion of the earth is not uniform, as well as for another cause, which is too intricate to mention here, the solar days are not, as we have said above, of equal length. So the device is adopted of ascertaining their average through the year and calling it the mean solar day. This, subdivided into hours, minutes, and seconds, is mean time—the clock-time of ordinary life.

If, therefore, it is desired to find correct time from a sun-dial, or by any method depending on the sun, the correction from apparent to mean time must be made. At four instants during the year this correction is zero. At other times a quantity, amounting at its greatest to about sixteen minutes, must be added to or subtracted from sun-time. For several days in the early part of November the sun is on the meridian more than a quarter of an hour before twelve o'clock. Our present system is not exact sun-time, but sun-time so modified as to be adapted to the current wants of our existence. It is uniform, because it is based on the time of revolution of the earth on its axis, which has not varied, if at all, more than one sixtieth of a second in the past twenty-five hundred years. But the common day is not the exact time of the earth's revolution, nor is the common year the exact time of its motion around the sun.

The tendency of civilization seems to be to depart from these strict