DAY, a word used with several different senses. Its most ancient meaning is the period of light («natural as opposed to, the period of darkness, and in this sense it is still.
qttite cotmnonly used. Its moSt common appli cation, however, is to the period of light and that of darkness together, but even in this. sense there are different days. The sidereal day is the time that elapses between two suc-. cessive culminations (see Cutuumnow) of any particular fixed star, or, in other words, is the time occupied by a revolution of the earth. round its axis. The solar, astronomical or ap parent day is the time that elapses between two; successive returns of the same terrestrial merid ian to the centre of the sun. This period is not always of the same length, and its mean length gives us the mean solar or civil day. The 24 hours of the sidereal day are numbered in succession from 1 to 24, while the civil day in most countries is divided into two portions of 12 hours each. The abbreviations P.M. and A.M. (the first signifying post meridiem, Latin for the latter ante meridiem, forenoon) are requisite, in' consequence of our division of the day into two periods of 12 hours each. In this respect the mode of numbering the hours from 1 to 24 consecutively has an advantage, and in some countries is being introduced; in parts of Italy it has long prevailed. The Baby lonians began the day at sunrise; the Jews and Greeks at sunset ; the Egyptians and Romans at midnight, as do most modern peoples. Astronomers use a day of the same length as the civil, but commonly make it begin at and number the hours up to 24, though latterly midnight has been partly adopted as the start ing point.
If we take a day according to the second definition given above (that is, a sidereal day), its length, of course, is the same throughout the year (see SIDEREAL TIME). The solar day, in consequence of the varying rapidity of the earth in its orbit, and the obliquity of the ecliptic, is different at different times (see SOLAR TIME), and this difference is uniform throughout the earth; but the time of the natural day (or period of light) is different at the different points of the earth, according to their distance from the equator. To get a fixed measure of solar time astronomers imagine a sun moving uniformly in the celestial equator and com pleting its circuit in the same time as the real sun. The time marked by this imaginary sun is called mean 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. Four times a year these two kinds of solar time coincide. In the intervals the sun is always either too fast or too slow, and the difference is called the equation of time. The daily apparent revolution of the sun takes place in circles parallel to the equator. If the equator and the ecliptic coincided the ,circle bounding light and darkness would always divide, into merely the equator, but all its par allels, into two equal parts, and the days and, nights would be equal in all the parallels through the year; but at the poles there would be no night. Owing to the inclination of the
earth's axis to the plane of its orbit (the eclip tic), the parallel of latitude in which the appears to move is continually changing; and therefore the equator alone (being a great cir cle) always remains bisected by the circle divid ing light from darkness; so that the days and nights here are always equal; while the parallels of latitude, not being great circles, are not i equally divided by the circle separating light from darkness, except at the time of the equi nox, when the sun is moving in the equator; and, of course, at this time only are days and nights equal in those parallels. As you approach the poles the inequality between the days and nights becomes continually greater, till, at the poles themselves, a day of six months alternates with a night of equal duration. The most dis tant parallel circles which the sun describes north and south from the equator are, as is well known, only 21Y2 degrees from it. The dis tance between the polar circles and the poles is the same. Therefore, as a little reflection will show, when the sun is in one of the tropics, all the polar circle in the same hemisphere will be within the illuminated region (because it will be within 90 degrees of the sun) during the whole of a diurnal revolution, while the other polar circle will be in the region of darkness. These circles, therefore, have one day of 24 hours and one night of the same length in each year. From the polar circles to the poles the time of the longest day increases fast, and in the same measure the length of the longest night. Not withstanding the inequality of the periods of light and darkness in the different parts of the earth each portion of the earth's surface has the sun above its horizon every year precisely six months, and below it the same length of time.
A day, in law, includes the whole 24 hours from midnight to midnight. In reckoning pe riods of time from a certain event the day on which the event occurred is excluded. On the other hand, if it be required to prove survival for a certain number of days, it will suffice if the person be alive for any portion, however small, of the last day. While an obligation to pay on a certain day would therefore be theo retically discharged by payment before mid night, the law requires that reasonable hours be observed— for example, if the payment (as a bill) is at a bank or place of business, it must be within business hours.
A lawful day is a day in which there is no legal impediment to the execution of a writ that is, a day may be unlawful, dies non juridi cus, either by common law or specific statute. By common law Sunday is a day on which the service of a writ cannot legally be made. Other days have been made holidays by both State and Federal statute in this country, and no such legal holiday is a lawful day.
Days of time at which a note is at maturity is in general three days after the time expressed on the face of it; the addi tional days are called days of grace. These days must be lawful days.