DAY, according to the most natural and obvious sense of the word, signifies that space of time during which it continues to be light, in contradistinction to night, which is that portion of time during which it is dark; but the space of time in which it is light being somewhat vague and indeterminate, the time between the rising and the setting of the sun is usually looked on as the day, and the time which lapses from its setting to its rising again as the night.

The word day is often taken in a large sense, so as to include the night also, or to denote the time of a whole apparent revolution of the sun round the earth; in which sense it is called by some a natural day, and by others an artificial one. But, to avoid confusion, it is usual to call it in the former sense simply the day, and in the latter a nyctherneron, by which term the latter acceptation is aptly denoted, as it implies both day and night. The nycthemeron is divided into twenty-four parts, called hours ; which are of two sorts, equal, and unequal or temporary.

Different nations begin their day at different hours. Thus the Egyptians began their day at midnight; and from them Hippocrates introduced that way of reckoning into astronomy, which Copernicus and others have followed; but the greater part of astronomers reckon the day to begin at noon, and so count twenty-four hours till the noon of the next day, and not twice twelve, according to the vulgar computation. The method of beginning the day at midnight prevails in Great Britain, France, Spain, and most parts of Europe.

The Babylonians began their day at sunrise, reckoning the hour immediately before its rising again the twenty-fourth hour of the day; whence the hours reckoned in this way are called the Babylonic hours. In several parts of Germany they begin their day at sunset, and reckon on till sunset next day, calling that the twenty-fourth hour ; these are generally termed Italian hours. The Jews also began their nycthemeron at sunset; but then they divided it into twice twelve hours as we do, reckoning twelve for the day, be it long or short, and twelve for the night; so that their hours continually varying with the day and night, the hours of the day were longer than those of the night during one half year, and the contrary during the other; whence their hours are called temporary. Those at the times of the equinoxes, however, became equal, because then those of the day and night were so. The Romans also reckoned their hours after this manner, as do the Turks at the present day.

This kind of hours is called planetary, because the seven planets were anciently looked upon as presiding over the affairs of the world, and as taking it by turns each of these hours, according to the following order; Saturn first, then Jupiter, Mars, the Sun, Venus, Mercury, and last of all the Moon; and hence they denominated each day of the week from that planet whose turn it was to preside over the first hour of the nycthemeron. Thus, assigning the first hour of Saturday to Saturn, the second will fall to Jupiter, the third to Mars, and so the twenty-second of the same nycthemeron will fall to Saturn again, and therefore the twenty-third to Jupiter, and the last to Mars; so that on the first hour of the next day it will fall to the Sun to preside, and by the same manner of reckoning the first hour of the next day will fall to the Moon, of the next to Mars, of the next to Mercury, of the next to Jupiter, and of the next to Venus; and hence the days of the week came to be distinguished by the Latin names of Dies Saturni, Solis, Lunoe, Martis, Mercurii, Jovis, and Veneris, and amongst us by the names of Saturday, Sunday, Monday, &c.