Fig. 95.
(3) The sun, moving along the ecliptic, always moves in a great circle, while the point of the meridian which is to overtake the sun moves in a diurnal circle, which is sometimes a great circle and sometimes a small circle. When the sun is at the equinoxes, the point of the meridian which is to overtake it moves in a great circle. As the sun passes from the equinoxes to the solstices, the point of the meridian which is to overtake it moves on a smaller and smaller circle: hence, as we pass away from the celestial equator, the points of the meridian move slower and slower. Therefore, other things being equal, the meridian will gain upon the sun most rapidly, and the days be shortest, when the sun is at the equinoxes; while it will gain on the sun least rapidly, and the days will be longest, when the sun is at the solstices.
The ordinary or civil day is the mean of all the solar days in a year.
81. Sun Time and Clock Time.—It is noon by the sun when the sun is on the meridian, and by the clock at the middle of the civil day. Now, as the civil days are all of the same length, while solar days are of variable length, it seldom happens that the middles of these two days coincide, or that sun time and clock time agree. The difference between sun time and clock time, or what is often called apparent solar time and mean solar time, is called the equation of time. The sun is said to be slow when it crosses the meridian after noon by the clock, and fast when it crosses the meridian before noon by the clock. Sun time and clock time coincide four times a year; during two intermediate seasons the clock time is ahead, and during two it is behind.
The following are the dates of coincidence and of maximum deviation, which vary but slightly from year to year:—
February 10 True sun fifteen minutes slow.
April 15 True sun correct.
May 14 True sun four minutes fast.