Editing Hipparchus (section)

=== Coordinate System ===
It is disputed which coordinate system(s) he used. Ptolemy's catalog in the ''[[Almagest]]'', which is derived from Hipparchus's catalog, is given in [[ecliptic coordinate system|ecliptic coordinates]]. Although Hipparchus strictly distinguishes between "signs" (30° section of the zodiac) and "constellations" in the zodiac, it is highly questionable whether or not he had an instrument to directly observe / measure units on the ecliptic.{{r|hoffman-befunde}}{{sfn|Hoffmann|2017}} He probably marked them as a unit on his celestial globe but the instrumentation for his observations is unknown.{{r|hoffman-befunde}}

[[File:Alm signs+consts.jpg|thumb|upright=1.8|Ptolemy's constellation areas (blue polygons) and "signs" of the zodiac had different sizes and extends; it is highly likely Hipparchus considered these units the same. Reconstruction from the Almagest{{sfn|Hoffmann|2017}}]]

Delambre in his {{lang|fr|Histoire de l'Astronomie Ancienne}} (1817) concluded that Hipparchus knew and used the [[equatorial coordinate system]], a conclusion challenged by [[Otto Neugebauer]] in his ''History of Ancient Mathematical Astronomy'' (1975). Hipparchus seems to have used a mix of [[ecliptic coordinate system|ecliptic coordinates]] and [[equatorial coordinate system|equatorial coordinates]]: in his commentary on Eudoxus he provides stars' polar distance (equivalent to the [[declination]] in the equatorial system), right ascension (equatorial), longitude (ecliptic), polar longitude (hybrid), but not celestial latitude. This opinion was confirmed by the careful investigation of Hoffmann{{sfn|Hoffmann|2017}} who independently studied the material, potential sources, techniques and results of Hipparchus and reconstructed his celestial globe and its making.

As with most of his work, Hipparchus's star catalog was adopted and perhaps expanded by Ptolemy, who has (since Brahe in 1598) been accused by some<ref>{{cite book |last1=Newton |first1=Robert Russell |title=The Crime of Claudius Ptolemy |date=1977 |publisher=Johns Hopkins University Press |location=Baltimore, MD |isbn=978-0-8018-1990-2 |url=https://archive.org/details/crime-of-cladius-ptolemy |via=[[Internet Archive]]}}</ref> of fraud for stating (''Syntaxis'', book 7, chapter 4) that he observed all 1025 stars—critics claim that, for almost every star, he used Hipparchus's data and precessed it to his own epoch {{fraction|2|2|3}} centuries later by adding 2°40' to the longitude, using an erroneously small precession constant of 1° per century. This claim is highly exaggerated because it applies modern standards of citation to an ancient author. True is only that "the ancient star catalogue" that was initiated by Hipparchus in the second century BC, was reworked and improved multiple times in the 265 years to the Almagest (which is good scientific practise even today).{{r|hoffmann2018}} Although the Almagest star catalogue is based upon Hipparchus's, it is not only a blind copy but enriched, enhanced, and thus (at least partially) re-observed.{{r|hoffman-befunde}}