The Winged Chariot of Time

     Solar Time 

        The apparent motion of the Sun across the sky has long been used as a basis for measuring time. At any locality, when the Sun reaches the highest point in the sky during any given day, it is noon. The north-south line across the sky through this point is the meridian. The interval between successive passages of the Sun across the same meridian is one day, and this day is by custom divided into 24 hours. The length of the day according to solar time is not the same throughout the year, however, because the apparent motion of the Sun varies. The difference in the length of the 24-hour day at different seasons of the year can amount to as much as 16 minutes. With the invention of accurate timepieces in the 17th century, this difference became significant. Mean solar time was invented, based on the motion of an imaginary Sun traveling at an even rate throughout the year.

 Standard Time 

        Standard time, which is based on solar time, was introduced in 1883 by international agreement to avoid the complications in railway schedules when each community used its own local solar time. The Earth was divided into 24 time zones. The base position is the zero meridian of longitude, which passes through the Royal Greenwich Observatory in southern England, and time zones are described by their distance east or west of Greenwich. Within each time zone all clocks are set to the same time. In the scientific model on which standard time zones are based, each zone spans 15° of longitude; in fact, however, the borders of time zones are bent to conform with international frontiers and the boundaries of regions within countries, as well as to facilitate commercial activities. In navigation, clocks are often set to the local time at Greenwich, called Greenwich mean time (GMT). Astronomers use essentially the same system but call it universal time (UT).

 Sidereal Time 

        Because mean solar time is based on the motion of a fictitious Sun, a base position was established from which the mean time can be calculated. This base position is the vernal equinox, an imaginary point in the sky. Practically, the location of the vernal equinox is found by reference to the position of the fixed stars. Solar time based on the position of the stars is called sidereal time, and a clock regulated to record sidereal time is called a sidereal clock. A discrepancy exists between the total number of hours in a mean solar year and those in a mean sidereal year. The Earth returns to the vernal equinox every 365 days 6 hr 9 min 9.54 sec by mean sidereal time. By mean solar time, the Earth returns every 365 days 5 hr 48 min 45.5 sec; the difference is 20 min 24.04 sec.

 Ephemeris Time 

        Neither mean solar nor mean sidereal time is precisely correct, because the motion of the Earth on its axis is not regular. Variations in the rate of rotation amount to 1 or 2 seconds per year. In addition, the Earth is gradually slowing down at the rate of about 1/1,000 of a second every 100 years. Some of these variations can be taken into account; others cannot because they are irregular. These difficulties were bypassed in 1940 when ephemeris time was introduced. Ephemeris time is used chiefly by astronomers when the greatest degree of accuracy is required in computing the positions of planets and stars. Ephemeris time is based on the annual revolution of the Earth around the Sun, and the base position, as in sidereal time, is the vernal equinox. Through the use of mathematical tables, ephemeris time is converted into mean solar time.

      Astronomical Time 

        Astronomical chronology is based on celestial phenomena and laws. The dates of celestial phenomena can be determined quite accurately by mathematical computation. By reckoning backward, the date of a historical event can often be verified or determined with precision if it was associated with an astronomical event, such as a solar eclipse. 

     Geological Time

        Covering the earth's entire history, beginning some 4.6 billion years ago, the scale of geologic chronology is second only to astronomical time, which covers the age of the universe. Geologists of the last century had scant evidence for calculating the age of the earth and its materials, arriving at widely disparate estimates of from 3 million to 500 million years, based on the rate of accumulation of sedimentary deposits. Relying mainly on stratigraphic correlation, using fossils and other kinds of evidence, they succeeded in constructing a relative time scale. But this first attempt at geologic chronology was of little use for comparing the stratigraphy of one continent with another, and without such comparisons the history of the earth remained largely an enigma. Then the discovery of radioactivity changed all this by laying the foundation for radiometric dating. Such dating techniques have since made it possible to calculate the absolute age of a mineral or rock and thereby date the age of the earth as well as events of the remote geologic past with an unprecedented degree of certainty. 

     Archaeological Time

        Although advanced civilizations are often dated by political chronology, some simple cultures are dated in ways more akin to the chronological methods used in geology. Thus, archaeologists carefully note the order of successive deposits (stratigraphy) containing human artifacts. The principle of stratigraphy assumes that in undisturbed strata the younger (more recent) layers overlie the older (earliest) layers, a relation often referred to as the law of superposition. The stratigraphic method of archaeological dating parallels that used in geology. In both methods, the thickness of deposits is one of the variables in time determination. Occasionally in the case of an ancient people, graves or artifacts occur in such a manner that actual geochronological methods help determine the age of the deposit. A widely used method for dating human cultures is the radiocarbon (carbon-14, or C14) technique. Based on the fact that living organisms take up a naturally occurring radioactive form of carbon during their lifetimes, this method permits investigators to determine how much radiocarbon still remains in an organic specimen found with any cultural remains. This determination relates directly to the time elapsed since the specimen was buried, and relatively accurate dates may be worked out from such information. 

     Political or Historical Time

        Political chronology is determined by the dates and the sequence of events in human history. Most ancient nations related their history to the lifetime of some central figure or to the reign of a king. This system gave a fairly complete chronology of an individual's time, but the history of the nation was often unrecorded between the death of a king and the accession of his successor and by the omission of obscure or unpopular kings from the written records. As political chronology developed, historians instituted the use of so-called eras based on national, ecclesiastical, or scientific reasons, each era being dated from an outstanding event or a convenient date called the epoch of that era. 

©1999 ThinkQuest Team 28906