Who the F*** Invented Time...or the Measurement of Time??? - a podcast by Garner Higgins

Apparent solar time is the time based directly on the Sun's position in the sky. In ordinary life the day runs from midnight to midnight. It begins when the Sun is invisible by being 12 hours from its zenith. Astronomers use the so-called Julian Day, which runs from noon to noon; the concept was invented by the astronomer Joseph Scaliger, who named it after his father, Julius. To avoid the problems caused by leap-year days and so forth, Scaliger picked a conveniently remote date in the past (4713 B.C.) and suggested just counting days without regard to weeks, months, and years. The reason for having the Julian Day run from noon to noon is the practical one that astronomical observations usually extend across the midnight hour, which would require a change in date if the astronomical day, like the civil day, ran from midnight to midnight.

Mean solar time, rather than apparent solar time, is the basis for local civil and standard time. The mean solar time is based on the position of a fictitious “mean sun.” The reason why this fictitious sun has to be introduced is the following: Earth turns on its axis regularly; it needs the same number of seconds regardless of the season. But the movement of Earth around the Sun is not regular because Earth's orbit is an ellipse. This has the result that Earth moves faster in January and slower in July. Though it is Earth that changes velocity, it looks to us as if the Sun does. In January, when Earth moves faster, the apparent movement of the Sun looks faster. The mean sun of time measurements, then, is a sun that moves regularly all year round; the real Sun will be either ahead of or behind the mean sun. The difference between the real Sun and the fictitious mean sun is called the equation of time.

Time zones. But if all clocks were actually set by mean solar time we would be plagued by a welter of time differences that would be “correct” but a major nuisance. A clock on Long Island, correctly showing mean solar time for its location (this would be local civil time), would be slightly ahead of a clock in Newark, N.J. The Newark clock would be slightly ahead of a clock in Trenton, N.J., which, in turn, would be ahead of a clock in Philadelphia. This condition prevailed until 1884, when a system of standard time was adopted by the International Meridian Conference. Earth's surface was divided into 24 zones. The standard time of each zone is the mean astronomical time of one of 24 meridians, 15 degrees apart, beginning at the Greenwich, England, meridian and extending east and west around the globe to the International Date Line

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