Time determination (astronomy)

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The concept of time determination is understood in astronomy and astrogeodesy as equivalent to that of astronomical position determination .

While the focus is on position and angle measurement when determining the location - more accurately: determining the geographical or astronomical latitude and longitude - when determining the time it is the precise observation of star passages . Time is understood here as the sidereal time of the location (of the observatory), the so-called local sidereal time Θ. It corresponds to the RA coordinate of all those stars that are currently passing through the meridian plane of the observer. Stars outside the meridian are characterized by their current hourly angle (see below), which changes by 15.04107 ° per hour due to the Earth's rotation (1 Earth's rotation in 23: 56: 04.091 hours).

history

The Greek poet Hesiod (farmer in the Boeotian village of Askra, around 700 BC) describes the right times for sowing, harvesting, seafaring, etc. by means of star phase phenomena such as morning first and morning last . In ancient Egypt analogous determinations were made by the heliacal rise of Sirius , with which the annual Nile flood was announced.

The exact time , however, was not important in everyday life in ancient times . It was sufficient to count the hours from sunrise - either as temporal hours (12 each for day and night) or later as regular Babylonian hours . In some cultures one counted after sunset ( Italian hours ). These times were determined with simple sundials or with the gnomon . Water or hourglasses were used for shorter time intervals . More precise methods were only known to priest astronomers and scientists , of whom the astrolabe in particular was used to measure and calculate the heights of the stars and, in some cases, the nocturnal on the big or small wagon.

However , some early results suggest that experts were able to achieve the accuracy of a few minutes with these observations of the stars , the astronomical noon, the morning and evening distance , etc. , some early results: above all the first catalogs of stars around 150 BC. BC, but also the Egyptian year length of 365.25 days, which the Julian calendar adopted, as well as precise lunar cycles and orbital times of planets .

The first wheel clocks and tower clocks were made around 1300, but really accurate time measurements were not made until 1650 with the pendulum clocks and the introduction of the second hand . From then on, the achievable accuracy increased by a factor of 10 every century, until from 1940 to 1970 quartz clocks advanced from milliseconds to microseconds and today's atomic clocks even reach 10-15 .

Basic equations of time determination

The local sidereal time corresponds to the star coordinate right ascension ( RA or α ) plus the current hour angle t of the observed star according to

Basic equation: Θ = α + t

The difference between two local sidereal times is identical to the difference between their geographical (more precisely: astronomical) lengths λ 1 and λ 2 . Therefore, when two observatories carry out an astronomical time determination, they can determine their mutual position (more precisely: the angle between their meridian planes) by comparing their clocks :

λ 2 -λ 1 = Θ 2 - Θ 1

If, instead of one of the local sidereal times, the one on the Greenwich meridian itself is taken (which is now easily possible thanks to the worldwide time signal services ), the length λ i of the observatory results directly . (Attention: this value is not the geographical longitude , but its astronomical equivalent related to the true vertical direction : the angle of the meridian plane to that in Greenwich , or the east-west difference angle of the vertical directions).

Traditional length difference measurement between observatories

Using this method, numerous observatories were measured relative to one another up to the 19th century by carefully bringing their chronometers to a reference point. The last time this took place was Albrecht 's length compensation around 1900. The more precise pendulum clocks , however, were not allowed to be transported. The invention of telegraphy made this laborious way of comparing clocks superfluous, while that of easily transportable quartz clocks was temporarily made practicable again. Today, time comparisons are made by radio, with LORAN or TV control signals, or directly in the system of radio time signals or GPS satellites. Only sometimes - when it comes to particularly high accuracies - atomic clocks are brought to a common location for comparison.

Since 1980, there were still some major measurement campaigns for an astronomical length compensation, among others, the European Danjon -Astrolab and some Zirkumzenitalen between the towns of Munich , Vienna , Graz and the fundamental station Wettzell , the accuracies in the range of milliseconds services (corresponding to an angle error only 0.01 "). They were part of a program for the international networking of reference stations in order to be able to connect the individual high-precision astrogeoids exactly to one another. Later other observatories were included in the network, only a few observatories in the Balkans (including Belgrade ) had to be due to the war At the Belgrade Astronomers' Congress in 2003 it was suggested that this should be done, but this seemed out of date to other experts in view of satellite geoid programs such as GRACE .

Determination of the local sidereal time

There are a number of standard and many special procedures for determining the local time , which can be varied depending on the local conditions (instruments and time system , geographical location, limited landscape horizon, refraction, cloud cover ...). The most important are:

Legal situation in Germany

In Germany the right to determine the time according to Article 73, Paragraph 1, No. 4 of the Basic Law belongs solely to the Federation. Time in Germany was regulated by the law on time determination until July 12, 2008 and has since been regulated by the Units and Time Act.

See also

Individual evidence

  1. ↑ Hesiod's use of star phases to determine time. (PDF 1.3 MB) Recklinghausen observatory, archived from the original ; Accessed December 30, 2015 (page 2, original website no longer available).