Astronomical Chronology

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The astronomical chronology (short also astro-chronology ) is an interdisciplinary subject between the chronology and the astronomy . It deals on the one hand with the astronomical basics of the calendar and the time , on the other hand with the dating of earlier astronomical and historical events.

Significant help in this process are the calendar calculations , the celestial mechanics and the archaeoastronomy , supplemented by occasional statements from the range of history , linguistics , arithmetic , physics , geodesy and other humanities or natural sciences.

Astronomical basics

The fundamental quantities of chronology are:

Day length and important time scales

The length of the day is not constant because the speed of the earth's rotation is gradually decreasing (currently at 0.002 seconds per century). This prompted scientists to introduce special time measures in the 20th century, four of which are mentioned here:

  • The world time was for the practical coexistence of people coordinated at the world as UTC (for time universal, coordinated , in England and GMT (for Greenwich Mean Time called)). It is based on the earth's rotation and a leap second is adjusted every 1–3 years .
    • The small difference to the actual phase UT1 of the earth's rotation is called dUT1 ; this time difference is a maximum of 0.9 s and is irrelevant for the chronology (as well as the pole movement ).
  • The ephemeris time ET refers to the very even annual orbit of the earth around the sun; it was introduced in 1960 for calculations in the solar system.
    • ET is based on the SI second, which was determined from the mean rotation of the earth from 1900 to 1905. It was replaced in 1984 by the Terrestrial Time TT , which emerged from it seamlessly.
    • As a result of the slowing down of the earth's rotation, the difference ΔT = TT – UT1 between 1900 and today (as of January 2020) has increased to over 69 s. TT is now more than a minute ahead of coordinated universal time.
    • Therefore, the planetary movements are measured with the more uniform dynamic time , the information in the Astronomical Yearbook is in TT (earlier ET).
  • The atomic time or French. TA (Temps Atomique) , internationally coordinated to TAI , is based (like all current time systems) also on the SI second and is used in physics and technology . According to the definition, TT - TAI = 32.184s has been valid since 1984 , but this could one day change slightly for reasons of atomic physics .
  • The GPS time . It has been running without any leap second since 1980, so that today (January 2020) it is 18 seconds ahead of UTC.

Calendar: year and month

There is a bewildering plethora of different calendar systems around the world that relate to the earth's annual orbit around the sun or the orbit of the moon . Without going into the cross-connections and historical entanglements, the following can be stated:

  1. In everyday life, the Gregorian calendar applies almost worldwide, the counting of which is also used in cultures with their own calendar;
  2. Caution should be exercised for times before October 15, 1582 (the date of the last calendar reform);
  3. The historians use of periods prior to 1582 the well-known, proleptically - Julian calendar of the Christian era, ie without Year Zero .
  4. For astronomers , the Gregorian calendar with a year zero would be more convenient, but its centuries are 36,524 or 36,525 days long. Therefore, a continuous day count from −4712 ( Julian date = JD) is used as the time scale . For example, noon on January 1, 2006 has the day number JD = 2,453,737 (2.453737 million days since the beginning of the year of 4713 BC). Longer-term calculations - such as precession - are usually calculated in Julian centuries of 36,525 days. However, you should be careful with calculations that go back beyond JD 0, as not all formula sets are valid for a negative Julian date.
  5. Modifications of the Julian date are widespread in IT and can therefore also be used as a calendar exchange format.

The only thing that alleviates this supposed - but practicable for every subject - "confusion" is the continuous days of the week . Therefore, for centuries, all attempts to reduce the year to exactly 52 weeks (364 days with 1–2 “weekless” final days), or even to introduce a 10-day week, were doomed to failure.

The following calendars are important for the evaluation of contemporary sources:

See also: List of Calendar Systems

Precession and nutation

See nutation (astronomy)

Historical sources of astronomical chronology

Worldwide there are a large number of astronomical buildings from prehistory that can be used for purposes of chronology. They include:

Essential aids are the alignment of the buildings according to (then) cardinal points , according to the rising and setting points of the sun and bright stars , the connection between any pictorial representations and their location or the time of origin, and much more.

Furthermore, finds and chronicles are to be cited, the content of which can be correlated with phenomena in the starry sky :

Here it is usually easier to establish the connections between the described phenomenon and the observation time and / or the position of the observer.

See also: spherical astronomy , astronomical phenomenology , orbit determination .

Important astronomical phenomena of the past

Only rarely are chronicles - those in the Orient up to about 4000 BC. Dating back to BC - contains general astronomical facts. It is more common that special phenomena are the reason for an entry. They include:

See also: Category: Astronomical event

Methods of astronomical chronology

The method of astronomical dating is closely related to the possibility of calculating the movements of the celestial bodies back into the past with sufficient precision. This brings several subject areas into play:

Important calculation methods of classical chronology are described in specialist literature, for example in the work of Paul Ahnert (see below ).

Examples

In the following, the wide range of astrochronology is demonstrated using two extreme examples:

For further examples, which are presented in detail in other articles in Wikipedia, see also:

  • The star of Bethlehem : The closer dating of the birth of Christ , provided that the star of the wise men was a close encounter between Jupiter and Saturn.
  • The Nebra Sky Disc : An approximately 3,600 year old bronze disc obviously related to astronomy.

Solar eclipse of 136 BC Chr.

Mesopotamian chronicles record a total solar eclipse on April 15, 136 BC. BC , whose central line ran exactly over Babylon. The reason why the fall is interesting is that it is independent of the shift in the vernal equinox in the precession cycle . The very reliable data allow the speed of the earth's rotation to be extrapolated over two millennia into the past.

If one of the today valid orbital elements of the Earth back calculated and the moon and the current axis rotation, you get a dark line through Mallorca . The 4000 km to the actually eclipsed Babylon are an expression of the fact that the earth's rotation has slowed down by about 1/30 of a second since then. Because this adds up to each of the almost 800,000 days, the result is 3¼ hours. The previously faster rotating earth means that the moon's shadow did not hit the surface of the earth before Spain, but in the Orient. On average, the Earth's rotation slowed by one to two milliseconds per century.

Computerized models and simulations

With the modern means of computer technology - such as computer simulation , numerical integration , equilibrium - and other model calculations  - many phenomena can be calculated back more precisely (and also faster) than with the classical , mathematically strict formulas of physics or celestial mechanics.

Simulation calculation: encounter and interaction of two galaxies

As an example, the partial image of a simulation is shown in which the encounter of two galaxies and their consequences were calculated from mutual gravity . This is not about millennia , but about millions of years . The picture comes from the article “ Extragalactic Objects ” and demonstrates how the billions of stars of the two spiral nebulae may fly past each other, but after all are allowed to form a common system.

Problems with such simulations include:

  • The choice of the most realistic possible starting data
  • The fineness of the model (specifically: how many stars are grouped together). If it is too coarse, the informative value suffers; conversely, the computing time increases enormously
  • The step size in time. If it is too short, the individual step becomes more precise, but the computing time increases and the rounding errors increase over millions of years.
  • Neglected interactions (e.g. thermal, relativistic), unknown dark matter, etc.

Simulations in the solar system are also calculated in a similar way - for example for star coverages, eclipses and planetary constellations.

See also

Other relevant phenomena:

Body of the solar system

various

Literature and web links

Individual evidence

  1. ↑ Until 1991 it was called TDT (Terrestrial Dynamical Time).
  2. ↑ Time scales. IERS , accessed January 8, 2020 .