This is exact as far as it goes, but the thought seizes one, how did it come about that man attempted to measure time? He saw the sunrise and he watched the fading sunset till "Hesperus with the host of heaven came," and the night melted again into the dawn. Nature marked definitely the hours of light and hours of darkness. That was a law over which he had no control. Similarly he watched the seasons--the spring, the summer, the autumn, and the winter; this gave him the annual calendar. It becomes a matter of curious speculation how it came to pass that man divided the year into twelve months, and how he came to give a name to each day, and to determine seven as forming a week. Similarly one is curiously puzzled as to why he divided day and night into twenty-four parts, calling them hours. These speculations lead us farther afield than the scope of this volume. An examination of Babylonian and Greek measurements of time is too abstruse to be included in a volume of this nature. Nor is it necessary, however interesting such may be, to record the astronomical observations at Bagdad of Ahmed ibn Abdullah. We must commence with the known data that the earth revolves on its axis in twenty-four hours, or, to be more exact, in 23 hours 56 minutes 4 seconds. Astronomical clocks recording with scientific exactitude this phenomenon are on a plane apart, as are chronometers used by mariners. The astronomer uses a clock with numbers on its dial plate up to twenty-four; the common clock has only twelve hour numerals. To come straight to modernity, it must be recognized that the measurement of time scientifically and the measurement of time according to civil law are two different things. The mean Solar day used in the ordinary reckoning of time, by most modern nations, begins at midnight. Its hours are numbered in two series from 1 to 12--the first series, called A.M. (_ante meridian_), before midday, and the second series, P.M. (_post meridian_), after midday. This is a clumsy arrangement and leads to confusion. The leading railways of the world are beginning to use the series of twenty-four. Let it be granted that the day consists of twenty-four hours, which is the apparent Solar day; the starting-point was not always the same. The Babylonians began their day at sunrise, the Athenians and Jews at sunset, the ancient Egyptians and Romans at midnight. In passing, it should be noted that the day is measured astronomically by recording the period of the revolution of the earth on its axis, determined by the interval of time between two successive transits of the sun, the moon, or a fixed star over the same meridian. The Solar day is exactly 24 hours, the Lunar day is 24 hours 50 minutes, and the Sidereal day is 23 hours 56 minutes.