Time Measurement, Time Zones, and the International Date Line

Updated March 9, 2021 | Infoplease Staff

How we structure time

The two natural cycles on which time measurements are based are the year and the day. The year is defined as the time required for Earth to complete one revolution around the Sun, while the day is the time required for Earth to complete one turn upon its axis. Earth needs 365 days plus about 6 hours to go around the Sun once, so a year does not consist of a round number of days; the fractional day has to be taken care of by an extra day every fourth year.

But because Earth, while turning upon its axis, also moves around the Sun, there are two kinds of days. A day may be defined as the interval between the highest point of the Sun in the sky on two successive days. This, averaged out over the year, produces the customary 24-hour day. But one might also define a day as the time interval between the moments when a certain point in the sky, say a conveniently located star, is directly overhead. This is called:

Sidereal time. A sidereal day is the time that it takes the Earth to complete one rotation on its axis so that a particular star can be observed twice at the meridian that runs directly overhead. Because the Earth is moving around the Sun as it rotates on its axis, the sidereal day is about 4 minutes shorter than the solar day, being equivalent to 23 hours, 56 minutes, and 4 seconds in mean solar time. As a result, a star will appear to rise about 4 minutes earlier every night, and different stars will be visible at different times of the year. Astronomers use a point that they call the “vernal equinox” to determine local sidereal time.

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 (as explained in the section The Seasons) 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. (This system was actually put into use a year earlier by the railroad companies of the U.S. and Canada, who, until then, had to contend with some 100 conflicting local sun times observed in terminals across the land.)

For practical purposes, this convention is sometimes altered. For example, Alaska, for a time, consisted of four of the eight U.S. time zones: the Pacific standard time zone (east of Juneau) and the 6th (Juneau), 7th (Anchorage), and 8th (Nome) zones, encompassing the 135°, 150°, and 165° meridians, respectively. In 1983, by act of Congress, the entire state (except the westernmost Aleutians) was united into the 6th zone, Alaska standard time.

The eight U.S. standard time zones are: Atlantic (includes Puerto Rico and the Virgin Islands), eastern, central, mountain, Pacific, Alaska, Hawaii-Aleutian (includes all of Hawaii and those Aleutians west of the Fox Islands), and Samoa standard time.

The Date Line. While the time zones are based on the natural event of the Sun crossing a meridian, the date must be an arbitrary decision. The meridians are traditionally counted from the meridian of the observatory of Greenwich, in England, which is called the zero meridian. The logical place for changing the date is 12 hours, or 180°, from Greenwich. Fortunately, the 180th meridian runs mostly through the open Pacific. The Date Line makes a zigzag in the north to incorporate the eastern tip of Siberia into the Siberian time system and then another one to incorporate a number of islands into the Hawaii-Aleutian time zone. In the south there is a similar zigzag for the purpose of tying a number of British-owned islands to the New Zealand time system. Otherwise, the Date Line is the same as 180° from Greenwich. At points to the east of the Date Line the calendar is one day earlier than at points to the west of it. A traveler going eastward across the Date Line from one island to another would not have to reset his watch because he would stay inside the time zone, but it would be the same time of the previous day.

See the world and U.S. time zone map,