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Introduction Travel geography Time Zones 6.1 Introduction: . Before clocks were invented and time read, people watched the sun to determine day and night. Primitive man planned his activities according to the different position of the sun. Because everyone wants to measure their day with the sun being at its highest point at midday, scientists came up with the idea of dividing the earth into different time zones. The time is the same everywhere within one time zone, but is different to all the other time zones. Travel geography Time Zones 6.1 Introduction: . There are 24 hours in a day, so there are 24 time zones. There are 360 degrees of longitude on the earth. Scientists have divided these by 24, that means that there are 15 degrees of latitude in every time zone. When you move from one time zone to the next one, you change your watch by one hour. If you are travelling in an easterly direction you move your watch one hour forwards, and if you are moving in westerly direction you move it one hour backward. Travel geography Time Zones 6.2 Greenwich Mean Line: . The planet is divided into 360⁰ of imaginary lines which run vertically from pole to pole. These lines are called Meridians, at 0⁰, passes through Greenwich, England. The Greenwich Meridian Line, Longitude 0⁰ is the center of world time. Time zones are determined by how many degrees east or west of the Prime Meridian one is , up to 180⁰ , which is on the opposite side of the planet, and the location of the International Date Line. Cont….. Travel geography Time Zones 6.2 Greenwich Mean Line: . The prime meridian was defined by the position of the large “Transit Circle” telescope in the Observatory’s Meridian Building in 1884. The Greenwich Meridian separates east from west in the same way that the Equator separates north from the south. But its position is marked in hundreds of other places too. The meridian passes through England, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and Antarctica. Travel geography Time Zones 6.3 Greenwich Mean Time (GMT): . GMT is a time zone at the longitude 0 degree 0 minutes the Prime Meridian at Greenwich, in the united Kingdom, has served as the reference line for GMT since the last 19th century. Time zones are determined by how many degrees east or west of the Prime meridian one is, up to 180⁰, which is on the opposite side of the planet, and the location of the International Date Line. This is where time changes a whole day. GMT was adopted as the world’s time standard at Washington Meridian Conference in 1884, also established 24 hour time-zone system. Travel geography Time Zones 6.4 Universal Time: . GMT is a term that is commonly referred to as Coordinated Universal Time (UTC). GMT was at first calculated by the 24 hour clock starting at noon. However, since 1925, the day of GMT starts at midnight. GMT was later renamed to Universal Time or UT. It is also known as UTO. UTC is the primary standard by which the world regulates clocks and time. For most common purposes, UTC is synonymous with GMT, but GMT is no longer precisely defined by the scientific community. Cont….. Travel geography Time Zones 6.4 Universal Time: . UTC is based on International Atomic Time(TAI), a time standard calculated using a weighted average of signals from atomic clocks located in nearly 70 national laboratories around the world. The only difference between the two is that UTC is occasionally adjusted by adding a leap second in order to keep it within one second of UT1, which is defined by the earth’s rotation. In the 40 years up to and including 2011, a total of 25 leap seconds have been added, the most recent was added on 30 June 2012. Travel geography Time Zones 6.5 Time Zones: . Militaries, large international companies and banking systems all use GMT as a reference when dealing with other parts of the world. Before 1972, all time zones were specified as an offset from GMT. Since 1972 all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes leap seconds to keep it within 0.9 seconds of GMT, now called UT1. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, Including UK. UTC is also called as Zulu Time By astronomers. Cont….. Travel geography Time Zones 6.6 International date Line: . It is an imaginary line of longitude on the earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian, in the middle of the pacific Ocean and makes two consecutive calendar days. To avoid crossing nations internally the line deviates around the far east of Russia and then various island groups in the pacific. These various deviations generally accommodate the political and /or economic affiliations of the affected areas. The time difference between either side of the IDL is not always exactly 24 hours because of local time zone variations. Travel geography Time Zones 6.7 Daylight Saving Time: . The relationship between GMT and other time zones may change, but only because those other time zones are adjusted for Daylight Saving Time. When DST is not observed, it is called standard time, normal time or winter time. It is also called “Summer time”. Many countries, and sometimes just certain regions of countries, adopt daylight saving time during part of the year. This involves advancing clocks by an hour near the start of spring and adjusting back in autumn. Purpose is to make better use of daylight. Travel geography Time Zones 6.8 Calculating Time: . One way to calculate time is to use world time. The twenty-four time zones correspond to meridians of longitude. Each zone is spaced 15 degrees apart and differs from its adjacent time zone by one hour. Universal Time or UT was designed to provide a single time keeping method, a single time zone that everyone would understand and which would avoid the confusion of local time zones. When converting times from one time zone we must take into account the time zone difference and also the effects of daylight savings. Cont….. Travel geography Time Zones 6.8 Calculating Time: . Some basic principles would apply namely that if you take the time at the Prime Meridian and you add time for locations to the west of it and subtract time for locations to its east. The number of time zones is equal to the number of hours to add or subtract to the coordinated universal time at the Prime meridian. Travel geography Time Zones 6.9 India Standard Time (IST): . India’s Time Zones were established in 1884. Originally there were two Time Zones, The Bombay Time and Calcutta Time. Calcutta time was 5 hours,30 minutes and 21 seconds in advance of GMT, while Bombay Time was 4 hours and 51 minutes ahead of GMT. Bombay continued to have a different time (39 minutes behind IST) until 1955. Indian Standard Time came into existence in 1905. Travel geography Time Zones 6.9 India Standard Time (IST): . Indian Standard Time is 5:30 hours ahead of Greenwich Mean Time. Indian Standard Time is the time observed throughout India. India does not observe daylight saving Time or other seasonal adjustments. Indian Standard Time is calculated on the basis of 82.5⁰ E longitude, from a clock tower in Mirzapur (25.15⁰ N 82.58⁰ E) near Allahabad in the state of utter- Pradesh which is nearly on the corresponding longitude reference line..
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