Saturday, December 20, 2014

Time Zones, Standard Time, Coordinated Universal Time and Daylight Saving Time

Time Zones: Because Earth is a rotating sphere, the Sun shines on only half of its surface at any one time. Thus, simultaneously morning, evening, and nighttime in different parts of the world, because of these disparities, each country or part of a country adheres to a local time. A region of Earth’s surface within which a single local time is used is called a time zone. The world is divided into 24 time zones by means of 24 standard meridians of longitude, each 15o apart. Time is measured in each zones as so many hours ahead or behind coordinated Universal Time (UTC). Countries or parts of countries falling in the vicinity of each zone adopt its time as shown on the map. Thus, when it is 12:00 noon UCT in London, it will be 2:00 pm in Zambia. 

Standard Time: Standard Times is the official local time in a particular country or part of a country. It is defined by the time zone or zones associated with that country or region. Although time zones are arranged roughly in longitudinal bands, in many places the borders of a zone do not fall exactly on longitudinal meridians, as can be seen on the map, but are determined by geographical factors or by borders between countries or parts of countries. Most countries have just one time zone and one standard time, but some large countries (such as the US, Canada and Russia) are split between several time zones, so standard time varies across those countries. For example, the coterminous United States straddles four time zones and so has four standard times, called the Eastern, Central, Mountain and Pacific Standard times. China is unusual in that just one standard time is used of the whole country, even though it extends across 60o of longitude from west of east.

Coordinated Universal Time: Coordinated Universal Time (UTC) is s reference by which the local time in each time zone is set. For example, Australian Western Standard Time (the local time in Western Australia) is set 8 hours ahead of UTC (it is UTC+8), whereas Eastern Standard Time in the United States is set 5 hours behind UTC (it is UTC -5). UTC is a successor to, and closely approximates, Greenwich Mean Time (GMT). However, UTC is based on an atomic clock, whereas GMT is determined by the Sun’s position in the sky relative to the 0o GMT longitudinal meridian, which runs through Greenwich, UK. 

Daylight Saving Time: Daylight saving is a summer time adjustment to the local time in a country or region, designed to cause a higher proportion of its citizens’ waking hours to pass during daylight. To follow the system, timepieces are advanced by an hour on a pre-decided date in spring and reverted back in the fall. 

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