DAY (AS. day, feel, days, Both. days, OHG. tar, Ger. Tag: probably connected with Skt. dal, Lith. degti, to burn, OPruss. dagas, summer). Originally the space of time during which it is light, in contrast to the space of darkness or night. It now more usually denotes a complete alternation of light and darkness. It is the earth's rotation that causes these changes of light and darkness. The earth being a globe, only one-half of it can be in the sun's light at once; that half enjoys daylight, while the other half in its own shadow, or in night; and by the earth's rotation the several portions of the surface have each their turn of light and of darkness.
Astronomers recognize more than one kind of day. if the time is noted when a particular fixed star is exactly south or on the meridian, when the same star collies again to the meridian the next (lay, the earth has made exactly one rotation, and the time that has elapsed is called a sidereal day. This portion of time is always of the same length; for the motion of the earth on its axis is strictly uniform, and is perhaps the only strictly uniform motion ;that nature presents to us. Sidereal time. or star time, front its unvarying uniformity. is much used by astronomers. But the passage of a star across the meridian is not a conspicuous enough event for regulating the movements of men in general. It is not a complete rotation of the earth, but a complete alternation of light and darkness. that constitutes their day. This, which is called the ciril or the solar clay, is measured between two meridian passages of the slut, and is about four minutes longer than the sidereal day. The cause of the greater length is this: When the earth has made one complete turn, so as to bring the meridian of the place to the same position among, the fixed stars as when it was noon the (lay before, the sun has in the mean time (apparently) moved eactward nearly one degree among the stars, and it takes the earth about four minutes more to move round so as to overtake it. if this eastward motion of
the sun were uniform. the length of the solar day would be as simple and as easily determined as that of the sidereal. But the ecliptic or sun's path crosses the earth's equator, and is therefore more oblique to the direction of the earth's rota tion at one time than another: and besides. as the earth moves in its orbit with varying speed, the rate of the sun's apparent motion in tile ecliptic. which is caused by that of the earth. must also vary. The consequence is that the length of the solar day is constantly fluctuating: and, to get a fixed measure of solar time, as tronomers have to imagine a sun moving uni formly in the celestial equator, and completing its circuit in the same time as the real sun. The time marked by this imaginary sun is called ?neon solar time: when the imaginary sun is on the meridian, it is mean noon; when the real sun is on the meridian it is apparent noon. It is that a sun-dial must show apparent time. while clocks and watches keep mean time. Only in four days of the year do these two kinds of solar time coincide. in the intervals the sun is always either too fast or too slow; and the differenee is called the equation of time (q.v.), because, when added to or subtracted from ap parent time, it makes it equal to mean time. The mean solar day is divided into 24 hours, the hours into minutes and seconds. A sidereal day, we have seen, is shorter: its exact length is 23 hours, 5(3 minutes. 4 seconds of mean solar or common time. Astronomers divide the side real day also into 24 hours. which are of course shorter than common hours. In the course of a civil year of 365 days, the earth turns on its axis 366 times• or there are 366 sidereal days. As tronomers reckon the solar day as beginning at noon, and count the hours from 0 to 23. The civil day begins at midnight, and the hours are counted in two divisions of twelve each. The ceciasiastieal day wa formerly reckoned from sunset to sunset. See INTERNATIONAL DATE LINE; TIME, STANDARD.