EQUATION OF TIME. It will be seen from the article EQUATION OF TUE CENTER (q.v.) that the earth's motion in the ecliptic—or what is the same thing, the sun's appar • ent motion in longitude—is not uniform. This want of uniformity would of itself obviously cause an irregularity in the time of the sun's coming to the meridian on suc cessive days; but besidep., this want of uniformity in the sun's apparent motion in the ecliptic, there is another cause Of inequality in the time Of its coming on the meridian viz., the obliquity of the ecliptic to the equinoctial. Even if the sun moved in the equinoctial, there would be an inequality in this respect, owing to its want of uniform motion; and even if it moved uniformly in the ecliptic, there would be such an inequality, owing to the obliquity of its orbit to the equinoctial. These two independent causes conjointly produce the inequality in the time of its appearance on the meridian, the correction for which is the equation of time.
When the sun's center comes to the meridian, it is apparent noon, and if it moved uniformly on the equinoctial, this would always coincide with mean noon, or 12 o'clock on a good solar clock. But from the causes above explained, mean and apparent noon
differ, the latter taking place sometimes as much as 10} minutes before the former, and at others as much as 14-i minutes after. The difference for any day, called, as we have said, the equation of time, is to be found inserted in ephemerides for every day of the year. It is nothing or zero at four different times in the year, at which the whole mean and unequal motions exactly agree—viz., about the 15th of April, the 15th of June, the 31st Aug., and the 24th December. At all other times, the sun is either too fast or too slow for clock-time. In the ephemerides above referred to, the sign + or — is prefixed to the equation of time, according as it is to be added to or from the apparent time to give the mean time. See NAU TICAL ALMANAC.