We normally keep time by the Sun, not the stars. That is, ordinary human timekeeping is based on mean solar time. Here "mean" means that we use a clock
that runs at a constant speed. The actual Sun is slightly ahead of its mean position at some times of year, and slightly behind at other times, because the Earth's orbit is not a perfect circle.
The mean solar time at longitude 0° (Greenwich, England) is called Greenwich Mean Time (GMT), Universal Time (UT), or sometimes, in military parlance, Z time or Zulu time.
The Earth is divided into time zones about an hour wide, each of which uses the mean solar time of a particular longitude. For example, the entire U.S. Eastern time zone uses the mean solar time for longitude 75° west. Table 2.1 describes the time zones of Britain, Canada, and the United States. For the rest of the world, see Figure 2.7.
The difference between standard time and local mean solar time is exactly 4 minutes per degree of longitude. For example, in Atlanta, at longitude
Table 2.1 Time zones of Britain, Canada, and the United States
52.5° W 60° W 75° W 90° W 105° W 120° W 135° W 150° W
Greenwich Mean Time = UTC British Summer Time = UTC+1h Newfoundland Standard Time = UTC—3h30m Newfoundland Daylight Time = UTC—2h30m
Atlantic Standard Time = UTC—4h Atlantic Daylight Time = UTC—3h Eastern Standard Time = UTC—5h Eastern Daylight Time = UTC—4h Central Standard Time = UTC—6h Central Daylight Time = UTC—5h
Mountain Standard Time = UTC—7h Mountain Daylight Time = UTC—6h Pacific Standard Time = UTC—8h Pacific Daylight Time = UTC—7h Alaska Standard Time = UTC—9h Alaska Daylight Time = UTC—8h
Hawaii Standard Time = UTC—10h
84° west, the local mean solar time is 36 minutes behind Eastern Standard Time because Atlanta is 9° west of the longitude on which the time zone is based.
Most localities observe daylight saving time (summer time) by putting their clocks ahead one hour in the summer. This can be interpreted as an attempt to hold sunrise nearer a fixed time of day, so that the increased length of summer days shows up mainly in the evening.
Time zones UTC+12 and UTC—12 are 24 hours apart; they meet at the International Date Line in the Pacific Ocean. They ought to be the beginning and end of the whole system, but in fact there is also a zone UTC+13 (New Zealand Summer Time), and UTC+14 is used in parts of the Kiribati Republic.
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