Diminishes to the west of the Prime Meridian. As a result, if a person moves east of the Prime Meridian, the time will be 12 hours shorter than at 0 degrees longitude. The time would be 12 hours longer for someone heading westward than the Prime Meridian.
Time is measured by means of clocks based on the position of Earth around its axis. Because we are not dealing with exact measurements, but rather estimates made every day based on observations done over many years, there is some error involved. This error increases as we get farther from the Equator, because more distant points on Earth's surface see a smaller fraction of the total area of Earth's surface. Error also increases with height above sea level because of the influence of air pressure.
Time zones were introduced so that people could have a common time even though they were not all located in exactly the same place. By assigning different times to places that differ by one hour of longitude, we can make sure that events such as sunrises and sunsets happen at the same moment across the world. Time zones were originally defined as areas of constant solar time (CSS), which was established by astronomers to help them coordinate their observations across different parts of the planet. Today, most countries use GMT/UTC instead, but they still define their time zones in terms of their UTC offsets.
The prime meridian, at 0 degrees longitude, is the starting point for all other longitude measurements. Time zones rise sequentially by one hour as you move east from the prime meridian, and drop consecutively by one hour as you move west. So if it's 9 a.m. in Los Angeles, it's 7 a.m. in Honolulu (nine hours ahead), and 3 p.m. in London (three hours behind).
During world war II, when most of Europe was without electrical power, they used atomic time instead. This meant that during that period, Paris was one second ahead of Greenwich mean time.
In 1945, when commercial electricity returned to Paris, they resumed their regular time zone on August 24th at 2 a.m., since there were no more than two hours of daylight at any time of year. So France stayed on UTC+1 until 1991, when they finally moved to UTC+0, which is what everyone else in Europe does today.
And even though most of North America is within one hour of Greenwich Mean Time, some places are further east or west than others. In 1941, when Canada became fully independent from Britain, they decided to keep Eastern Standard Time, which at the time was five hours behind GMT. Western Canada, including most provinces west of Ontario, kept Pacific Standard Time, which was then equivalent to our current GMT-7 time zone.
Traveling from one longitude to another takes 4 minutes. In general, a one-degree change in longitude results in a four-minute difference. The places east of the Prime Meridian are ahead of the places in the western hemisphere in terms of time. California and Hawaii are about 9 minutes ahead of London; New York is about 5 hours behind.
Longitude lines are used to divide up the world into equal parts. The first longitude line was drawn by Gerardus Mercator in 1569 when he divided the world into two right-angled triangles with one side along the equator. Before then the only way to divide up the world was politically - France and Spain divided up Latin America while Germany and England divided up Europe. Mercator's map became the standard reference for all future maps until the advent of modern satellites in the 1960s.
In order to keep track of which part of the world is currently day and which part is currently night, we need to know what time it is in different places. The International Earth Rotation and Reference Systems Service (IERS) publishes daily estimates of earth rotation parameters such as longitude and latitude at many observatories around the world. These estimates are used by scientists studying earthquakes, volcanoes, climate change, and other topics related to geophysics. IERS also provides weekly estimates of terrestrial elevation changes from radar studies of landmasses around the world.
Longitudes or meridians are imaginary lines that run vertically between the poles of the Earth. Each of these longitudes is referred to as a degree. Per longitude (each degree) has a time difference of 4 minutes. So, if it was 12:04 p.m. in Greenwich (0 degrees), it would be 12:04 p.m. at 1 degrees, and so on. The farther north you go, the later it is given the same time zone.
Time zones are used for convenience reasons; they allow people to sleep during daylight hours and get more done during the night. Time zones were first established by the British in 1884. Before then, local sun times were used instead.
In practice, the difference between living in London and New York is not so much the time it takes for a letter to reach them but how they use those letters. In London, decisions are made quickly, often over a cup of coffee, and things tend to happen. In New York, decisions can be made slowly, over several cups of coffee, and things take longer to get done.
These are just two examples of how usage matters more than location in terms of creating differences in time. There are eight time zones used across the United States and one time zone used in Canada. Alaska uses the Pacific Time Zone but is three hours behind Utah, which makes it nine hours behind California, which makes it three hours ahead of Arizona, which means it stays in PDT until 9 p.m., at which point it switches over to MST.