Time Scales (Zeitmaßstäbe)

Im Folgenden nehme ich Bezug auf meinen Eintrag vom 6. Dezember 2008 (Projekt G.O. Mueller) in diesem Blog:

Über „die Zeit“  –  eine Ausnüchterung

Wie aus nachstehender Studie „Time Scales“  hervorgeht, gibt es aus historischer Sicht eine Vielzahl von Definitionen für Zeitmaßstäbe, die für das Verständnis des Zeitbegriffes wichtig sind.


Time Scales

The concept of time has been refined throughout history, and new understanding usually produces a new time scale. The list in this web page aims mostly at describing when and why new time scales were developed.  At the end of this web page are links to other web pages which better explain the definitions of various time scales. I also have some plots of some of these time scales.

Earth rotation time

Historically the most obvious indicator of the passage of time has been the diurnal cycle of earth rotation. The day is the fundamental element of all calendars.

The rotation of the earth under the moon and sun produces tides. Reckoning time by tides is sufficiently simple that rocks can do it. Sedimentary rocks called tidal rhythmites contain records of earth paleorotation dating back three billion years.

Apparent Solar Time
Local Time — sometimes LT

Apparent Solar Time has been used since prehistory. It is reckoned at any location by observation of obvious phenomenon such as sunrise, sunset, or passage through the meridian (noon). Reckoning time by sunrise and sunset is sufficiently simple that plants can do it.

Reckoning time by local apparent noon is sufficiently simple that anyone with a stick in the ground can do it. The ellipticity of the earth’s orbit and the obliquity of the earth’s equator produce a variation in the duration of a day reckoned by apparent noon. The variation is known as the „equation of time“, and it is large enough that a pendulum clock is stable enough to measure it. The „equation of time“ is best visualized in combination with the annual variation in the latitude of the sun in order to produce the analemma.

The earliest almanacs tabulated the „equation of time“. Until 1930 in the British Nautical Almanac and 1935 in the American Ephemeris and Nautical Almanac the position of the sun was tabulated at apparent noon as well as at mean noon. 

Mean Solar Time — sometimes MST

The principles behind mean solar time and the „equation of time“ were known to Ptolemy. By the middle of the 19th century personal timepieces were sufficiently accurate and sufficiently widespread that civilization began to run on mean solar time instead of apparent solar time. In this case „mean“ means the average obtained by considering the rotation of the earth in the absence of the annual variations caused by obliquity and eccentricity. Telegraphs were particularly strong drivers of mean solar time because they began to require synchronization of human activities over large distances.

Julian Day Number

A decimal integer count of consecutive days beginning on January 1, 4713 B.C in the proleptic Julian calendar. The Julian Period of 7980 years is a combination of the 19-year Metonic cycle of 235 months, the 28-year cycle of the Julian calendar, and the 15-year cycle of the Roman indiction. The scheme was invented by Scaliger in 1582 who realized that all three cycles were in year 1 during that conveniently prehistoric year. (Note that the ISO 8601 format used elsewhere in this document is inappropriate here because it specifies the Gregorian calendar where the year is always positive.)

Julian Date — JD

In 1849 John Herschel published a treatise suggesting that astronomers (who preferred to use the same date for all of the observations of a single night) should adopt JD as an indication of the number of mean solar days (and decimal fractions thereof) elapsed since JD 0.0 which was at Greenwich mean noon of -4712 January 1 (using the astronomical reckoning of the years in the proleptic Julian calendar).

Modified Julian Date — MJD

In keeping with civil usage and the International Meridian Conference of 1884 where days are reckoned from midnight, and also for the sake of convenience of not handling such large numbers, the MJD was defined in the 1950s as ( JD – 2400000.5 ). MJD 0.0 corresponds to 1858-11-17T00:00:00. (Two URLs which attribute the origin of MJD to artificial satellite tracking at SAO are found in Austria and Australia.)

The IAU has had a love/hate/love relationship with MJD. In 1973 the IAU resolved that it should be used. In the mid-1990s the IAU seriously discussed the possibility of recommending that the term MJD should not be used because several other quantities with the same name but different definitions have been in use in various contexts. Finally (?) in Resolution B1 of the XXIIIrd General Assembly in 1997 the IAU recognized that when properly defined the term MJD may be used. (This may have been another case of capitulation to practical reality akin to that of the 1935 IAU resolution regarding GMT which basically admitted that no action by the IAU could prevent the use of the term.)

Subsequent to the creation of time scales other than Greenwich Mean Time, forms of JD and MJD expressed in those time scales have also been used to indicate elapsed time measured in ephemeris days and in multiples of 86400 SI seconds. As recommended by the IAU in 1997 any current application which requires precision better than one minute, or any historical application which requires precision better than several hours, should take care to indicate which time scale is associated with the use of JD or MJD.

It should also be noted that the use of JD or MJD for the UTC time scale is problematic and ambiguous at the precision of one second. JD and MJD express the elapsed count of some form of „day“ as real numbers along a presumably unsegmented, continuous number line. The UTC time scale (and, historically, GMT as used in practical situations before the advent of UTC) contains changes in rate and discontinuities. In particular, there is no obvious way to represent a leap second of UTC (or the smaller leaps present in the available forms of GMT and UTC before 1972) using JD or MJD notation.


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