Day

Day, in Astronomy, has two meanings. 1. The sidereal day, or the length of time taken for the earth to make exactly one rotation about its axis. This body performing a rotation of 360° in one sidereal day, it follows that such a period is the interval between consecutive passages of any distant star across the meridian. Sidereal time is much employed by astronomers. One result of the tides is to gradually lengthen the day; shrinkage of the earth due to cooling produces the opposite effect, and on these and other accounts the length of the sidereal day has remained wonderfully constant for ages. This interval is divided into 24 sidereal hours, and each such hour into minutes and seconds. 2. The solar day is the length of time between successive passages of our sun across any meridian. Inasmuch as we are travelling in an ellipse round the sun, our speed being greatest in winter when we are nearest the sun, the length of the solar day varies throughout the year. It is longest in winter and shortest in summer, but is always greater than the sidereal day on account of the earth's rotation round the sun being in the same sense as its rotation about its own axis. In ordinary usages solar time is desirable, for the use of sidereal time would involve such difficulties as that of 12 o'clock sidereal midday happening at 12 o'clock solar midnight. Thus we are compelled to use solar time; and since the solar day varies throughout the year, a mean solar day or average for the whole circuit is taken and subdivided into hours, minutes, and seconds. Such a day is about four minutes longer than the sidereal day. The duration of light and darkness in a day depends on the latitude and on the season. If we regard the day as the length of time that the sun is above the horizon, we may say that at the poles the days and nights are each of six months' duration. If the sun is vertically above the equator, as it is during the equinoxes (q.v.), the days and nights are of equal length; if the sun is north of the equator, as in our summer, the days in the northern hemisphere are longer than the nights, and vice versa