|
|
Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time. Before this, almost every town in the world kept its own local time. There were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be. However, with the vast expansion of the railway and communications networks during the 1850s and 1860s, the worldwide need for an international time standard became imperative.
What is UTC and all that stuff?
The times of astronomical and weather phenomena
and events that are observed internationally (by treaty, for example) are
often given in "Universal Time" (abbreviated (UTC) or "Greenwich Mean Time"
(abbreviated GMT). The two terms are often used synonymously to refer,
loosely, to time kept on the Greenwich meridian (longitude zero), five
hours ahead of Eastern Standard Time.
However, when a precision of one second or
better is needed, it is important to know that UTC and GMT have slightly
different meanings for astronomical phenomena and civil affairs, as explained
below. In either case, times given in UTC or GMT are almost always given
in terms of a 24 hour clock. Thus, 14:42 (often written simply 1442) is
2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a
time to indicate GMT, as in 0935Z.
However, in normal civil usage, UTC or GMT
refers to a time scale called "Universal Time, Coordinated" (abbreviated
UTC), which is the basis for the worldwide system of civil time.
The UTC time scale is kept by a large number
of highly precise atomic clocks at facilities around the world, including
the U.S. Naval Observatory, and there is international coordination in
maintaining UTC to better than a nanosecond (billionth of a second) per
day. The length of a UTC second is defined in terms of a count of radiation
cycles of a certain atomic transition of the element cesium, and is not
directly related to any astronomical phenomena.
What is a Meridian?
A meridian is a north south line selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the sky.
Hemispheres?
The line in Greenwich represents the Prime
Meridian of the World - Longitude 0º. Every place on Earth is measured
in terms of its distance east or west from this line. The line itself divides
the eastern and western hemispheres of the Earth - just as the Equator
divides the northern and southern hemispheres. East longitudes are positive,
west longitudes are negative.
| To obtain: | |
| Atlantic Daylight Time | Subtract 3 hours from UTC |
| Atlantic Standard Time | Subtract 4 hours from UTC |
| Eastern Daylight Time | Subtract 4 hours from UTC |
| Eastern Standard Time | Subtract 5 hours from UTC |
| Central Daylight Time | Subtract 5 hours from UTC |
| Central Standard Time | Subtract 6 hours from UTC |
| Mountain Daylight Time | Subtract 6 hours from UTC |
| Mountain Standard Time | Subtract 7 hours from UTC |
| Pacific Daylight Time | Subtract 7 hours from UTC |
| Pacific Standard Time | Subtract 8 hours from UTC |
| Alaska Daylight Time | Subtract 8 hours from UTC |
| Alaska Standard Time | Subtract 9 hours from UTC |
| Hawaii Aleutian Daylight Time | Subtract 9 hours from UTC |
| Hawaii Aleutian Standard Time | Subtract 10 hours from UTC |
| Samoa Standard Time | Subtract 11 hours from UTC |
Thus, 11:00 UTC is the same as 5:00 CST. When converting zone time to or from UTC, dates must be properly taken into account. For example, 10 March at 02:00 UTC is the same as 9 March at 21:00 EST. The table can also be used to determine the difference between the time observed in any two zones. For example, the table shows that Eastern Standard Time is three hours "ahead" of Pacific Standard Time.
