Calendopaedia - Time

Rotational time.

All methods for measuring the passage of time require counting the cycles of regularly occurring phenomena. The simplest and most universally used cycle is the rotation of the Earth. As the Earth turns, the stars and the sun appear to move in an arc across the sky, disappear under the horizon, reappear at the opposite horizon, and return to their original positions in the sky. A time system based on the apparent motion of the stars is called sidereal time. A sidereal day is the time it takes for a star to appear to make one complete circuit.

The period of the Earth's rotation with respect to the sun (from one high noon to the next) is called the solar day. A solar day is about four minutes longer than a sidereal day. This is so because as the Earth orbits the sun, the sun appears to move slowly eastward against the fixed stars. Thus, for an observer on Earth, it takes slightly longer for the sun to return to its original position in the sky than it takes for the stars.

Because the Earth moves faster in its orbit around the sun when it is close to the sun than when it is far away, the length of the apparent solar day is not constant throughout the year. To provide a more uniform time system, the mean solar day the annual average length of a solar day is used to establish mean solar time, to which we set our watches.

Standard time and time zones.

The local mean solar time at any location depends on where that place is on the globe: it advances by four minutes for each degree longitude to the east. To avoid confusion, most nations keep what is called standard time in established zones known as time zones. The world is divided into 24 time zones. The width of each is about 15 degrees longitude. By international agreement, the line of longitude running through Greenwich, England, was adopted as the prime, or zero, meridian. The time in this time zone is called Greenwich mean time (GMT).

The international date line is an imaginary zigzag line in the mid-Pacific Ocean that runs roughly along 180 degrees longitude. Travelers crossing it westward add a calendar day; travelers crossing it eastward lose a day.

An adjustment of regional standard time, called daylight saving time, was adopted by some countries to conserve fuel by reducing the need for artificial light in the evening hours. Clocks are advanced one hour in the spring and set back one hour in the autumn (fall).

Universal and ephemeris time.

In 1928 astronomers adopted the term Universal time (UT) for the mean solar time at the meridian of Greenwich, England. Later they defined several kinds of Universal time, the most accurate and uniform being UT2. Another time scale, ephemeris time (ET), is more uniform than UT2. It is based on the orbit of the Earth around the sun. This scale is not very practical, however, because accurate calculations require complex astronomical observations. In 1964 a new time scale, called coordinated Universal time (UTC), was internationally adopted and has now largely replaced Greenwich mean time as the universal standard of time. UTC is more uniform and more accurate than either the UT2 or ET systems because the UTC second is based on atomic time (see below), though the UTC year is still based on the time it takes the Earth to complete one orbit around the sun. Because the rate of the Earth's rotation is gradually slowing, it is necessary to add an extra second, called the leap second, to the length of the UTC year. This is usually done no more than once or twice a year.

Atomic time.

Today the length of the second as defined in the International System of Units is based on a specific number of transitions, or vibrations, in a particular kind of cesium atom. These transitions produce extremely regular waves of electromagnetic radiation that can be counted to produce a highly accurate time scale. Coordinated Universal time is based on this second, called the SI second.

The cesium-beam clock is the most accurate standard of atomic time currently in use, but scientists are working on using other kinds of atoms for atomic clocks. Such clocks based on hydrogen or beryllium atoms, for example could be thousands of times more accurate even than today's cesium clocks.

Many of the world's nations maintain very accurate cesium clocks. The time kept by these clocks is averaged together to produce what is called international atomic time (TAI). Time signals from the world's national-standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites. Highly accurate time signals are used for, among other things, tracking space vehicles and studying the motions of the Earth's crust.

Pulsar time.

In 1967 a new kind of star called a pulsar was discovered. These stars emit regular pulses of radiation many times per second. The regularity of these pulses has sparked interest in the possibility of a pulsar clock. Although the measurements involved are complex, it appears that some pulsars may be even more regular than atomic clocks.

Radiometric time.

Radioactive elements, such as uranium, decay spontaneously into other elements or isotopes. The time it takes for one half of the atoms in a sample of a particular radioactive element to decay is called the element's half-life. Each radioactive element has a different half-life, so in a sense these elements contain internal clocks that generate a kind of time known as radiometric time. Scientists use this principle to determine the approximate age of organic specimens by measuring the ratio within the specimen of a radioactive form of carbon to the stable form.

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