Daylight Saving Time Will End on November 1, 2009

Category: Image - :DaylightSaving-World-Subdivisions.png|300px|thumb|

alt=Colored world map. Europe, Russia, most of North America, parts of southern South America and southern Australia, and a few other places are colored blue (DST used). Most of equatorial Africa and a few other places near the equator are colored red (DST never used). The rest of the land mass is colored orange (DST no longer used). Oceans are white.|

Although not used by most of the world's people, daylight saving time is common in high latitudes.

Daylight saving time (DST; also summer time in British English?see Terminology) is the convention of advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Modern DST was first proposed in 1895 by George Vernon Hudson, a New Zealand entomologist. Many countries have used it since then; details vary by location and change occasionally.

The practice is controversial. Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, evening entertainment and other occupations tied to the sun. Traffic fatalities are reduced when there is extra afternoon daylight; its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited and often contradictory.

DST's occasional clock shifts present other challenges. They complicate timekeeping, and can disrupt meetings, travel, billing, recordkeeping, medical devices, heavy equipment, and sleep patterns. Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is commonly used in practice to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, especially by bodies connected with the United Kingdom, such as the BBC World Service,What is GMT? at the BBC Radio World Service the Royal Navy, the Met Office and others, although strictly UTC is an atomic time scale which only approximates GMT with a tolerance of 0.9 second. It is also used to refer to Universal Time (UT), which is a standard astronomical concept used in many technical fields and is referred to by the phrase Zulu time.

In the UK, GMT is the official time only during winter; during summer British Summer Time is used. GMT is substantially equivalent to Western European Time.Legal time in the UK is still "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978 (with an exception made for those periods when the Summer Time Act 1972 orders a hour's shift for daylight saving), see also the legal section in this article. Mean time at Greenwich was the prototype of Universal Time, as from the late 19th century, see the terms of the International Meridian Conference. Thus UT1, as a form of UT equalized for polar motion, is still a measure of mean time at Greenwich. The widely used time signals based on UTC are kept within 0.9 seconds of UT1, see 'History' section of this article). (The practice of referring to the time signals as 'GMT' is therefore only quantitatively correct to 0.9 second, and is informal and unofficial, see the 'time zone' section of this article.) UTC+0 is explained elsewhere in this article (links from map caption) as equal to Western European Time (WET). Thus WET is within 0.9 second of Greenwich mean time, and (within that tolerance) can fairly be described as 'substantially equivalent'.

Noon Greenwich Mean Time is not necessarily the moment when the noon sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Historically the term GMT has been used with two different conventions for numbering hours. The old astronomical convention (before 1 January 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours. The latter convention is modern practice (on and after 1 January 1925) for astronomical as well as civil purposes. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

An atomic clock is a type of clock that uses an atomic resonance frequency standard as its timekeeping element. They are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, and to control the frequency of television broadcasts and GPS satellite signals.

Atomic clocks do not use radioactivity, but rather the precise microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were masers with attached equipment. Currently the most accurate atomic clocks are based on absorption spectroscopy of cold atoms in atomic fountains such as the NIST-F1.

National standards agencies maintain an accuracy of 10-9 seconds per day (approximately 1 part in 1014), and a precision set by the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized, by using leap seconds, to UT1, which is based on actual rotations of the earth with respect to the mean sun.