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 observations, show only a small and rather doubtful change, amounting perhaps to less than one-hundredth of a second per century. As this amount seems to be markedly less than that which would be expected 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. The moon's apparent mean motion in longitude seems also to indicate slow periodic changes in the earth's rotation; but these are not confirmed by transits of Mercury, which ought also to indicate them. (See MOON and TIDES.) (S. N.)
_Legal Aspects._--In law, a day may be either a _dies naturalis_ or natural day, or a _dies artificialis_ or artificial day. A natural day includes all the twenty-four 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. In cases where action must be taken for preserving or asserting a right, a day would mean the natural day of twenty-four hours, but on the other hand, as in cases of survivorship, for testamentary or other purposes, it would suffice if a person survived for even the smallest portion of the last day necessary.
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 (e.g. the Parliamentary Elections Act 1868, the Corrupt and Illegal Practices Prevention Act 1883) Sundays and holidays are excluded in reckoning days, and consequently all the Sundays, &c., of a prescribed sequence of days would be eliminated. So also, by custom, the word "day" may be understood in some special sense. In bills of lading and charter parties, when "days" or "running days" are spoken of without qualification, they usually mean consecutive days, and Sundays and holidays are counted, but when there is some qualification, as where a charter party required a cargo "to be discharged in fourteen days," "days" will mean _working days_. Working days, again, vary in different ports, and the custom of the port will decide in each case what are working days. In English charter parties, unless the contrary is expressed, Christmas day and other recognized holidays are included as working days. A _weather working day_, a term sometimes used in charter parties, means a day when work is not prevented by the weather, and unless so provided for, a day on which work was rendered impossible by bad weather would still be counted as a working day. _Lay days_, which are days given to the charterer in a charter party either to load or unload without paying for the use of the ship, are days of the week, not periods of twenty-four hours.