Hipparchus (astronomer)

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Hipparchus (fantasy picture)

Hipparchus of Nicaea (Ἵππαρχος, dt. Hipparch , * around 190 BC in Nicaea ; † around 120 BC, probably in Rhodes ) was the most important Greek astronomer of his time. He is considered the founder of scientific astronomy and was also a geographer and mathematician . The astrometry satellite Hipparcos (High Precision Parallax Collecting Satellite) was named in his honor.

Hipparchus proceeded with the utmost precision in his research. When comparing his own studies of the sky with those of earlier (also Babylonian ) astronomers, such as Aristyllos and Timocharis , he discovered the slow precession or shift of the equinoxes . His calculation of the tropical year (the length of the year determined by the seasons ) deviates from modern measurements by only 6.5 minutes. Hipparchus invented a method to determine positions on earth using geographical latitude and longitude . He calculated the best star catalog to date with the locations and brightnesses of around 900 stars and designed the corresponding star map . Hipparchus also put together a table with trigonometric chords ( chord table ), which formed the basis of modern trigonometry .

Life and complete works

The best ancient source on Hipparchus is the Almagest of the Alexandrian astronomer Ptolemy (approx. 100–175). Further information can be found in the works of Pappos of Alexandria and Theon of Alexandria in the 4th century, in the "Geography" of Strabons and the "Naturalis historia" Pliny the Elder from the 1st century.

The dates of Hipparchus' life are unknown, but in the Almagest there are observations made by him from the years 147 to 127 BC. Chr. Handed down. Jean Baptiste Delambre calculated from this and from further evidence a birth around 190 BC. BC, in all probability in Nicaea in Bithynia . Further considerations lead to a death date around 120 BC. There are no known contemporary portraits; ancient coins with his image were not minted in Bithynia until the 2nd and 3rd centuries. Because it is known through Ptolemy that Hipparchus observed from Rhodes , the island is also assumed to be the place of death.

Hipparchus' main works are also lost. Only one critical commentary in two books on a well-known poem by Aratos by Soloi has survived. From later mentions it can be concluded that Hipparchus himself wrote a bibliography of his larger works, which consisted of about 14 works. His catalog of stars is incorporated into the Almagest's catalog, but Ptolemy does not name the sources of the measurements individually and in some cases not at all. Nonetheless, Hipparchus 'catalog can be at least partially restored, since the measurements he took over have a common systematic error due to the precession between Hipparchus' and Ptolemy's work.

Hipparchus made a celestial globe based on his measurements . Although the original was lost here too, it is known that this globe was copied. A copy of such a copy could rest on the shoulders of the Atlas in the Farnesian collections in Rome and represent one of the oldest surviving celestial globes.

Hipparchus is considered the father of scientific astronomy and, along with Ptolemy and Aristarchus of Samos , is considered to be one of the greatest astronomers of antiquity.

The 150 km large lunar crater Hipparchus and the asteroid (4000) Hipparchus are named after him.

Astronomical work


By comparing the position of the Spica , the main star of Virgo , with the star words measured about 150 years earlier , Hipparchus discovered the precession of the equinoxes , which had shifted by about 2 °. From this he calculated this rotation around the ecliptic pole with at least one degree per century; the actual value is one degree per 72 years. Hipparchus attributed the precession to a slow movement of the fixed star sphere. It was not until Copernicus recognized its existence by a conical movement of the Earth's axis. Both the position measured by Hipparchus and the previous position were determined during lunar eclipses . Since the time elapsed between lunar eclipses was known very precisely from Babylonian observations, Hipparchus came to the conclusion that the background of the starry sky must have shifted.

He also calculated the distance between the earth and the moon ( Lunar Distance ) to be 30 diameters of the earth (the exact mean distance of 384,400 km corresponds to about 30.17 diameters of the earth).

Astrometry and star catalog

Timocharis of Alexandria and Aristyllos had already around 300 BC. A compilation of asterisk words made, but it was probably incomplete. In 135 BC A new star appeared, possibly a supernova . Because at that time the fixed star sphere was considered to be eternal, such new stars were explosive - as in the times of Tycho Brahe . Hipparchus decided to create a more complete star catalog , which must have included between 800 and 1000 stars. The catalog itself has not survived, but it is very likely that Ptolemy took over large parts of the star positions in his Almagest and corrected them for the precession that had now accumulated. Since the latter was known only imprecisely, groups of presumably older measurements can be identified in the Almagest.

Criticism from Pliny

Two centuries later, the Roman natural philosopher Pliny (approx. 23–79) expressed a clear criticism of Hipparchus' star catalog - and at the same time proof of his working methodology . In addition to the nova, Hipparchus had also motivated a variable star to expand the catalog , Pliny says:

"Even Hipparchus ... has discovered a new star and another that formed in his time and was caused by its movement ... to think about whether this happens more often and whether the stars we thought were attached [to the celestial sphere] were also moving. And so he began a work contrary to God : namely to count the stars for the descendants and to record the constellations according to their names with invented tools to mark the locations and sizes of the individual stars ... Perhaps that among his spiritual heirs there was someone who would grow them [or lose weight]. "

On the one hand, Pliny 's reproach of godlessness shows how much the starry sky was considered eternal and immutable in Pliny' s time, while Hipparchus had compiled his catalog of stars centuries earlier, "so that later generations could derive the shifting of stars from it."
On the other hand, the text passage shows, how exactly Hipparchus must have determined the star brightness .

The length of the seasons

The different lengths of the seasons were already known to the Babylonians , but Hipparchus improved the values ​​considerably. In this way he created the basis for precise position measurements that followed the apparent path of the sun . The position measuring device introduced by Hipparchus and then generally used, the armillary sphere , was calibrated according to the sun.

Mathematical work

Hipparchus also compiled the first known table of trigonometric chords, which formed the basis for trigonometric calculations. In today's notation this is for the angle A (and the circle with radius 1)

Chord ( A ) = 2 · sin ( A / 2).

He gave the values in increments of 7.5 ° for A to.

See also

Individual evidence

  1. Nicolaus Copernicus: De revolutionibus orbium coelestium , 3rd book, chapter 1
  2. Ernst Zinner : The history of astronomy. From the first beginnings to the present. Julius Springer, Berlin 1931, pp. 125-126.
  3. Lucio Russo : The forgotten revolution or the rebirth of ancient knowledge. Springer, Berlin 2005, ISBN 3-540-20938-7 , p. 101.

Web links

Commons : Hipparchus  - collection of images, videos and audio files