Time

C

Standard Time

In 1883 an international agreement introduced the concept of standard time. Standard time was adopted to avoid the complications of adhering to railroad time schedules when each community used its own local solar time. The base position for standard time is the prime meridian. The distance east or west of Greenwich determines the standard time zone and, thus, the standard time of a particular location.

D

Dynamical Time

Astronomers use dynamical time for the precise study of the motion of celestial bodies. Dynamical time replaced ephemeris time in 1984, when the International Astronomical Union (IAU) updated the Astronomical Almanac. Scientists introduced ephemeris time in 1940 and selected the orbital position of Earth around the Sun as the standard by which to define the numerical measure of ephemeris time. In the 1950s the IAU decided that ephemeris time could be based on the orbital position of any planet or satellite. Time would be determined by comparing the orbital position of a particular planet or satellite (natural or artificial) at a particular time to an ephemeride. An ephemeride is a table of orbital positions of a planet or a satellite mapped over a period of time.

The annual revolution of Earth around the Sun is the basis for dynamical time, and the base position of measure (as in sidereal time) is the vernal equinox. When the greatest degree of accuracy is required in computing the positions of a planet or star, astronomers use dynamical time, because neither mean solar time nor mean sidereal time is sufficiently accurate, as the motion of Earth on its axis is not regular and even. Variations in the rate of Earth’s rotation amount to 1 or 2 seconds per year.

E

Atomic Time

Atomic time is the time scale of physics. Scientists use atomic time when they require exceptionally precise measurements of time intervals relating to physical phenomena. Clocks became more accurate and precise through the centuries, and with the introduction of atomic clocks—specifically, the construction of a high-precision cesium atomic clock in 1955—extremely accurate measurement of time became possible. Early mechanical clocks varied by several minutes each day. In the 1920s, vibrating quartz crystals were accurate to a few ten-thousandths of a second per day. The cesium atom clocks used in the 1980s lost less than a second in 3,000 years. In the 1990s the National Institute of Standards and Technology (NIST) in the United States established an atomic clock—the NIST-7, also a cesium clock—that is accurate to a single second over 3 million years. The electronic components of atomic clocks are regulated by the frequency of radiation emitted or absorbed by a particular atom or molecule.

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III

Scientific Standard of Time

Until 1955 astronomers and scientists calculated the scientific standard of time—the second—based on Earth's period of rotation. They defined the second as 1/86,400 of a mean solar day. When scientists realized that Earth's rate of rotation is irregular, a redefinition of the second became necessary. In 1955 the IAU defined the second as 1/31,556,925.9747 of the solar year that was in progress at noon on December 31, 1899. The International Committee on Weights and Measures adopted this definition in 1956. Since 1967 the official length of a second in the International System of Units (SI) has been defined by atomic standards: a second is equal to 9,192,631,770 oscillations, or periods, of the radiation corresponding to the transition between two hyperfine (closely spaced) energy states of the cesium-133 atom.

IV

Time Zones

For the purposes of standard time, Earth is divided into 24 standard time zones. The time zones extend from the North Pole to the South Pole, and within each zone the time is the same throughout. Within each time zone, local noon corresponds approximately to the time at which the Sun crosses the central meridian, or longitude, of that zone.

The distance east or west of the Greenwich meridian determines different time zones. According to the scientific model of standard time, each standard time zone spans 15° of longitude. In fact, the borders of time zones are bent to conform to state and country boundaries, as well as to facilitate commercial activities. In 1966 the U.S. Congress passed the Uniform Time Act, which established eight standard time zones for the United States and its outlying regions. In 1983 several time zone boundaries were altered so that most of Alaska, which formerly spanned four zones, could be unified under one time zone. The U.S. standard time zones are the Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, and Samoa zones.

There are five standard time zones in Canada. From east to west these are the Atlantic, Eastern, Central, Mountain, and Pacific time zones. Newfoundland has its own time zone, which is not a standard time zone. Newfoundland time is 30 minutes ahead of Atlantic time.

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