A short chronology of timekeeping

TIME AND SPACE ANALOGY

Chronology, science dealing with division of time into regular periods, the arrangement of events in order of their occurrence, the assignment of correct dates to known events, and the reconciling of discrepancies in dates caused by variations in the systems employed in modern and ancient times.

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.

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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.

Middle Eastern Chronology

In ancient Mesopotamia, home of Sumer, Assyria, and Babylon, continuous chronology begins with the birth of King Sargon I (reigned about 2335-2279 BC), and a fairly complete list of kings has been compiled to the first dynasty of Ur, extending back to about 2670 BC and possibly earlier. Chaldean astronomers discovered the cycle that modern astronomers call the Saros, consisting of a period of 223 lunation's (slightly more than 18 years) that is still important in the calculation of eclipses. The chronology of ancient Egypt begins with the reign (circa 3100-3066 BC) of Menes, first king of the 1st Dynasty. The Egyptian year began with the rising of the star Sirius, but, as their civil year contained exactly 365 days, the Egyptians were compelled to use the so-called Sothic cycle, a period of about 1460 years from one time to the next, when the civil year coincided with the astronomical one. Such a coincidence occurred about AD 140, but it is uncertain whether the first cycle started about 2780 or 4240 BC.

Greek and Roman Chronology

The era of the Olympiads of the Greeks was reckoned from July 1, 776 BC, and Greek astronomers introduced the Metonic cycle of 235 lunation's (almost exactly 19 years) and also the Callippic cycle of 940 lunation's (closer to 76 years). In Roman chronology the era of the founding of the city (ab urbe condita, or AUC) dates from April 22, 753 BC, and the Julian era dates from the reform of the calendar by Julius Caesar in 45 BC. In AD 312 Emperor Constantine the Great introduced a cycle of 15 years called indiction.

Western Chronology

The Christian era, now used almost exclusively throughout the Western world for civil chronology, was first used in 525 by the Christian monk Dionysius Exiguous, who fixed the birth of Christ in the year of Rome 753. It is generally agreed that this date should have been fixed some years earlier. Dionysius's chronology was introduced into historical writings by Bede the Venerable in the 8th century.

Scriptural chronology is extremely uncertain because various local chronologies were used at different times by scriptural writers, and different systems were used by contemporaneous writers. The Jewish Mundane era, beginning in 3760 BC, was not used until the 10th century AD. Jews now also use dates coinciding with those in the Christian era, but they treat the dates as belonging to a Common era and designate BC dates as BCE and AD dates as CE. The creation was dated by the 17th-century Irish archbishop James Usher as 4004 BC. The Muslim era dates from the Hegira, July 16, AD 622, but because the Muslim calendar is based on lunar months and is of variable length, the reconciliation of its dates with those of the Gregorian calendar is very complicated.

The epoch of the Christian era is too recent to be a convenient reference point for technical calculations. The French classical scholar Joseph Justus Scaliger proposed in 1583 that the epoch of the Julian era be fixed at January 1, 4713 BC, at noon. The beginnings of the cycles used in antiquity coincided at that time, and the date was sufficiently remote to furnish a reference point to which all other chronological systems might be compared. The Julian cycle contains 7980 years of 3653 days, but computation is seldom by years, and the days are numbered consecutively.

In many cases, astronomical chronology is used to verify or correct dates given in history for political events. For example, by reckoning backward, the time and place may be fixed for the occurrence of a remarkable eclipse, such as the eclipse reported by the Greek scientist Thales as causing the suspension of a battle between the Medes and the Persians. That eclipse was found to have occurred on May 28, 585 BC. Scottish history furnishes another instance: When King Håkon IV (the Old) of Norway sailed from Bergen with his Norse fleet to punish the king of Scotland, he landed in the Orkney Islands; there the sun appeared as a thin, bright ring. The British physicist Sir David Brewster found that an annular eclipse of the sun was visible in the Orkneys on August 5, 1263, about one o'clock. Such verifications in chronology depend on the testimony of contemporary writers or on information derived from inscriptions found on coins, medals, or monuments.

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In the history of Western civilizations, the principal problems of chronology are encountered in the reconciliation of dates in the various calendars, such as the Julian, Gregorian, and Muslim. For example, an article appeared in the Edinburgh Courant of February 19, 1706. The article was an abridgment of one published in the London Gazette of February 13, 1705, which, in turn, was a translation from the Amsterdam Gazette of February 22, 1706. All three were published in the same week. The discrepancy in year was caused by the fact that Scotland and the Low Countries began the year on January 1, while England, until 1752, began it on March 25. The discrepancy in days was caused by the use of the Gregorian calendar in the Low Countries, at a time when England and Scotland still adhered to the Julian calendar.

Jim Gray is keeper of the NBS-4 atomic clock at the National Institute of Standards and Technology. Some atomic clocks lose or gain only one second every 200,000 years.

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TIME

Time, period during which an action or event occurs; also, a dimension representing a succession of such actions or events. Time is one of the fundamental quantities of the physical world, being similar to length and mass in this respect. Three methods of measuring time are in use at present. The first two methods are based on the daily rotation of the earth on its axis. These methods are determined by the apparent motion of the sun in the sky (solar time) and by the apparent motion of the stars in the sky (sidereal time). The third method of measuring time is based on the revolution of the earth around the sun (ephemeris 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. 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 hr. 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 throughout the year. The difference in the length of the 24-hr day at different seasons of the year can amount to as much as 16 min. With the invention of accurate timepieces in the 17th century, this difference became significant. Mean solar time was invented, based on the motion of a hypothetical 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 that followed in railroad time 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 that passes through the Royal Greenwich Observatory, Greenwich, 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 state boundaries and international frontiers 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 possessions. In 1983 several time zone boundaries were altered so that Alaska, which had formerly spanned four zones, could be nearly unified under one time zone. The U.S. standard time zones are the Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, and Samoa zones. In navigation, clocks are often set to the local time at Greenwich, called Greenwich mean time (GMT). Astronomers use this system but call it universal time (UT).

Sidereal Time

Because mean solar time is based on the motion of a hypothetical sun, a base position from which the mean time can be calculated was established. This base position is the vernal equinox, an imaginary point in the sky that is, nevertheless, calculated with great accuracy by astronomers. 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. A clock regulated to record sidereal time is called a sidereal clock. The U.S. Naval Observatory in Washington, D.C., uses mathematical tables to derive mean solar time from mean sidereal time. The mean solar times thus calculated have a margin of error of 1 part in 1 million. A discrepancy exists between the total number of hours in a mean solar year and in a mean sidereal year because the earth is revolving around the sun at the same time it is rotating on its axis. According to mean sidereal time, the earth returns to the vernal equinox every 365 days 6 hr 9 min 9.54 sec. According to 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 accurate, because the motion of the earth on its axis is not regular. Variations in the rate of rotation amount to 1 or 2 sec per year. The rate has varied as much as 30 sec during the last 200 years. In addition, the earth is gradually slowing down at the rate of about 1/1000 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.

The Scientific Standard of Time

Until 1955, the scientific standard of time, the second, was based on the earth's period of rotation and was defined as 1/86,400 of the mean solar day. When it was realized that the earth's rate of rotation was irregular and also slowing down, it became necessary to redefine the second. In 1955 the International Astronomical Union defined the second as being 1/31,556,925.9747 of the solar year in progress at noon December 31, 1899. The International Committee on Weights and Measures adopted the definition the following year.

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With the introduction of atomic clocks—specifically, the construction of a high-precision cesium beam atomic clock in 1955—more accurate measurement of time became possible. This atomic clock relates the number of spectral lines caused by the cesium-133 atom, alternating between an excited state and a ground state, to standard measurements of time. In 1967 the measurement of the second in the International System of Units was officially defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

Time Dilation

Time is not a physical constant, although the passage of time in any one place can be measured with great accuracy and precision. The effect of motion and gravity on time is that it is dilated or expanded. These effects were formulated (1905) by Albert Einstein in his special theory of relativity and were observed in experiments conducted in the 1960s and '70s. In one such experiment in 1971, atomic clocks were carried on two high speed aircraft. One traveled eastward, that is, in the rotational direction of the earth, and one westward. After the flight, the onboard clocks were found to have either lost or gained time (relative to a ground based atomic clock) depending on their direction of travel, thus confirming a predicted effect of relativity.

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Analogy Clock of Time

This clock is design so that an average person can see how the measurement of time on an astronomical scale can be compared to time on a solar clock of one day. A 24 hour clock is a scale the average person can see and understand. To do this we must take all the time which has passed from the start of the known universe until today and compare it to the sweep of the hands on a 24 hour clock. The current believe is that the clock started 12 billion years ago. To represent this on a 24 hour clock then each hours sweep of the hand would represent 500 million years (1/2 billion years). On this clock the hand swept for about 15 hours before the earth, sun and the planets start forming thus starting geological time (4.5 billion year ago). Life on our planet started only after the clock had been running for 23 hours or about one hour ago (representing 500 million years ago), 28 seconds ago after the clock had been sweeping around for 23 hours 59 minutes and 32 seconds man has come out of the trees (5 million years ago) thus starting archaeological time, 3/4 of a second ago man has begun speaking and language is born and with it political and historical time (115 thousand years ago). On the clock of all known time, human history is not as long as one mississippi! Less than one second ago contains all usable humans knowledge. Hard concept to accept for a species which knows that the Universe revolves about him.

People cannot visualize the difference between millions and billions of anything. For example if I told you a million dollars in 100 dollar bills can fit in an attaché case and taken out stack on a table would be about 2 feet high could you tell me how high a billion dollar of 100 dollar bills would stack? If you told them that it would stack higher that the tallest building that exist on earth they could not believe it. It would stack over 2000 feet high or over a 200 story building. There is no comparison between one million and one billion anything. Next time you hear that all welfare recipients stole 27 million dollars and a stock broker swindled 27 billion from pension funds you will understand the damage a few white collar criminals can do to our system. Just try to graph these two numbers on a single sheet of paper.

Time is not a constant but is effected by motion and gravity.

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THINGS OF SPACE DESOLVE IN TIME ?

Time Line

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