The precise determination of time rests on astronomical and atomic definitions that scientists have established with the utmost mathematical exactness. Although scientists are able to describe the past and the future and demarcations such as seconds and minutes, they cannot define exactly what time is. The scientific study of time began in the 16th century with the work of Italian physicist and astronomer Galileo Galilei. In the 17th century English mathematician and physicist Sir Isaac Newton continued the study of time. ____________________________
Several ways to measure time are in use today. Solar time is based on the rotation of Earth on its axis. It makes use of the Sun's apparent motion across the sky to measure the duration of a day. Sidereal time is also based upon Earth's rotation, but uses the apparent motion of the "fixed" stars across the sky as Earth rotates as the basis for time determination. Standard time, the familiar clock time most people use in everyday life, is based on the division of Earth's sphere into twenty four equal time zones. Dynamical time (formerly called ephemeris time) is the timescale of astronomy. Astronomers use the orbit of Earth around the Sun, as well as the orbital motions of the Moon and the other planets, to determine dynamical time. Atomic time is gauged on the frequency of electromagnetic waves that are emitted and/or absorbed by certain atoms or molecules under particular conditions. It is the most precise method for measuring time currently possible... _________
Until the late 1800s most towns and cities set clocks based upon the rising and setting of the sun. Because of the earth's rotation, dawn and dusk occur at different times at different places, but time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications. With the growth of rapid locomotive travel and long-distance telegraph communications during the 1830s, schedule and message conflicts began to arise. As an example, because each train station set its own clock, it was difficult to coordinate train schedules. In the 1870s American railroads maintained 50 different time zones. Technology had created a need for a unified time-keeping system. Cleveland Abbe an American meteorologist, developed a system of weather reporting and forecasting using the telegraph to share information between weather stations. In order to compile his information, Abbe required a time-keeping system that was consistent between the stations. To accomplish this he divided the United States into four standard time zones. In 1883 Abbe convinced North American railroad companies to adopt his time zone system. In 1884 Britain, which had already adopted its own standard time system for England, Scotland, and Wales, helped gather international consensus for global time zones. Since the earth rotates 15 degrees of longitude per hour, the earth's 360 degrees were divided into 24 zones, each measuring about 15 degrees in width. The 0° longitude line, or meridian, was defined as a line running through the old Greenwich Observatory in Greenwich, England. Time in each of the twelve zones east of Greenwich increases one hour for each zone. Time in each of the twelve zones to the west of Greenwich decreases by one hour. The International Date Line lies at the 180° meridian on the opposite side of the earth from Greenwich and divides the eastern and western time zones. The time difference between each side of the International Date Line is 24 hours. Thus, a traveler heading west across the date line loses one day while a traveler headed east gains a day.
Cleveland Abbe an American meteorologist, developed a system of weather reporting and forecasting using the telegraph to share information between weather stations. In order to compile his information, Abbe required a time-keeping system that was consistent between the stations. To accomplish this he divided the United States into four standard time zones. In 1883 Abbe convinced North American railroad companies to adopt his time zone system. In 1884 Britain, which had already adopted its own standard time system for England, Scotland, and Wales, helped gather international consensus for global time zones. Since the earth rotates 15 degrees of longitude per hour, the earth's 360 degrees were divided into 24 zones, each measuring about 15 degrees in width. The 0° longitude line, or meridian, was defined as a line running through the old Greenwich Observatory in Greenwich, England. Time in each of the twelve zones east of Greenwich increases one hour for each zone. Time in each of the twelve zones to the west of Greenwich decreases by one hour. The International Date Line lies at the 180° meridian on the opposite side of the earth from Greenwich and divides the eastern and western time zones. The time difference between each side of the International Date Line is 24 hours. Thus, a traveler heading west across the date line loses one day while a traveler headed east gains a day.
Since the earth rotates 15 degrees of longitude per hour, the earth's 360 degrees were divided into 24 zones, each measuring about 15 degrees in width. The 0° longitude line, or meridian, was defined as a line running through the old Greenwich Observatory in Greenwich, England. Time in each of the twelve zones east of Greenwich increases one hour for each zone. Time in each of the twelve zones to the west of Greenwich decreases by one hour. The International Date Line lies at the 180° meridian on the opposite side of the earth from Greenwich and divides the eastern and western time zones. The time difference between each side of the International Date Line is 24 hours. Thus, a traveler heading west across the date line loses one day while a traveler headed east gains a day.
The time in any given time zone or country may shift by one hour for certain periods of the year to gain maximum daylight hours and balance these hours from morning to evening. One such system is Daylight Savings Time in the United States....
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