Standard of Time. In order to have a standard of time we must have a perfectly reliable and uniform motion as a means of measurement. The rotation of the earth on its axis is uniform and constant; hence equal intervals of time occur between any two successive passages of a fixed star across the meridian. If we divide the time between such passages into twenty-four hours, we shall obtain the sidereal hour; but this is by no means convenient in ordinary life. It is convenient in obtaining certain astronomical measurements, and a sidereal clock (q.v.) is to be found in every observatory; but a certain hour on that clock - say 8 oclock - may mean any time during the night or day, according to the time of year, and hence would be practically useless to people generally. On the other hand the period between two successive passages of the sun across the meridian - a solar day - is far more convement, and, if the sun travelled uniformly round the equator, it would be a perfectly simple thing to obtain the value of a solar hour. [TIME.] The sun, however, does not behave in this way; it moves in the ecliptic instead of the equator, and its motion is irregular. An imaginary sun is, however, supposed to move under these simple conditions and to perform its complete journey in the same tlme as the real sun; it, in fact, takes an average of the sun's motions, and the day obtained in this way is known as a mean solar day. A clock which neither loses nor gains is regulated to this mean time, while observations on the sun itself would give apparent time. The correction to be applied to the apparent time to convert it into mean time is known as the EQUATION OF TIME (q.v.), and is never greater than 16' 18".