DAY, in astronomy, the interval of time in which a revolu tion of the earth on its axis is performed. Days are distinguished as solar, sidereal or lunar, according as the revolution is taken relatively to the sun, the stars or the moon. The solar day is the fundamental unit of time in daily life and in astronomical prac tice. In the latter case, being determined by observations of the sun, it is taken to begin with the passage of the mean sun over the meridian of the place, or at mean noon, while the civil day begins at midnight.
The question of a possible variability in the length of the day is one of fundamental importance. One necessary effect of the tidal retardation of the earth's rotation is gradually to increase this length. It is remarkable that the discussion of ancient eclipses of the moon, and their comparison with modern ob servations, show only a small and rather doubtful change, amount ing perhaps to less than one-hundredth of a second per century. As this amount seems to be less than that which would be ex pected from the cause in question, it is probable that some other cause tends to accelerate the earth's rotation and so to shorten the day. (See MooN and TIDEs.) Legal Aspects.—In law, a day may be either a dies naturalis or natural day, or a dies artificialis or civil day. A natural day includes all the 24 hours from midnight to midnight. Fractions of the day are disregarded to avoid dispute, though sometimes the law will consider fractions, as where it is necessary to show the first of two acts or events. In cases where action must be taken for preserving or asserting a right, a day means the whole natural day of 24 hours.
When a statute directs any act to be done within so many days, these words mean clear days, i.e., a number of perfect intervening days, not counting the terminal days : if the statute says nothing about Sunday, the days mentioned mean consecutive days and include Sundays. Under some statutes Sundays and holidays are excluded in reckoning days, and consequently all the Sundays. etc., of a prescribed sequence of days would be eliminated. By custom, the word "day" may be understood in some special sense.
In England the period of the civil day may and does vary under different statutes. Daytime, within which distress for rent must be made, is from sunrise to sunset. An obligation to pay money on a certain day is discharged if the money is paid before mid night of the day on which it falls due, but the law requires reason able hours to be observed. If, for instance, payment has to be made at a bank or place of business, it must be within business hours.
When an act of parliament is expressed to come into operation on a certain day, it is to be construed as coming into operation on the expiration of the previous day. (Interpretation Act 1889, § 36; Statutes [Definition of Time] Act 1880.) Under the orders of the supreme court the word "day" has two meanings. For purposes of personal service of writs, it means any time of the day or night on week-days, but excludes the time from 12 midnight on Saturday till 12 midnight on Sunday. For purposes of service not required to be personal, it means before six o'clock on any week-day except Saturday, and before 2 P.M. on Saturday.
Closed Days, i.e., Sunday, Christmas day and Good Friday, are excluded from all fixtures of time less than six days : otherwise they are included, unless the last day of the time fixed falls on one of those days (R.S.C., 0. lxiv.).