Hipparchus (Greek Hipparchos) (circa 194 B.C.  circa 120 B.C.) was a Greek astronomer, mathematician and geographer.

Hipparchus was born in Nicaea (Greek Nikaia), ancient district Bithynia, (modernday İznik[?]) in province Bursa, today Turkey.
The exact dates of his life are not known for sure, but he is believed to have made his observations from 162 B.C. to 126 B.C.. The date of his birth (circa 190 B.C.) was calculated by Delambre, based on clues in his work. We don't know anything about his youth either. Most of what is known about Hipparchus is from Strabo's Geographica (Geography), from Pliny the Elder's Naturalis historia (Natural sciences) and from Ptolemy's Almagest. He probably studied in Alexandria.
His main original works are lost. His only preserved work is the (Commentary) on the Phaenomena of Eudoxus and Aratus or Commentary on Aratus, a commentary in 2 books on a poem by Aratus, which describes the constellations and the stars, which comprise them. This work contains many measurements of stellar positions[?] and was translated by Karl Manitius[?] (In Arati et Eudoxi Phaenomena, Leipzig, 1894). All his other works were lost in a burning of the Great Royal Alexandrian Library in 642.
For his accession he is recognized as originator and father of scientific astronomy. He is believed to be the greatest Greek astronomer observer and many regard him at the same time as the greatest astronomer of ancient times, although Cicero still gave preferences to Aristarchus of Samos. Some put on this place also Ptolemy of Alexandria.
Hipparchus had in 134 B.C. ranked stars in six magnitude classes according to their brightness: he assigned the value of 1 to the 20 brightest stars, to weaker ones a value of 2, and so forth to the stars with a class of 6, which can be barely seen with the naked eyes. This was later adopted by Ptolemy, and modern astronomers with telescopes, photographic plates and with other measuring devices for the light as they extended a luminosity with a density of light current j of a star on the Earth and put it on a qunatitative base. Observations with measuring devices for the light had shown that the density of light current of a star with a apparent magnitude 1^{m} is hundred times greater of a star with a magnitude 6^{m}. If we consider a property of an eye that a response is proportional with a logarithm of irritation, we get Pogson[?]'s physiological law (also called Pogson's ratio) from 1854 (other sources 1858):
Hipparchus had made a lot of astronomical instruments, which were used for a long time with nakedeye observations. About 150 B.C. he made the first astrolabion, which may have been an armillary sphere or the predecessor of a planar instrument astrolabe, which was improved in 3rd century by Arab astronomers and brought by them in Europe in 10th century. With an astrolabe Hipparchus was among the first able to measure the geographical latitude and time by observing stars. Previously this was done at daytime by measuring the shadow cast by a gnomon, but the way this was used changed during his time. They put it in a metallic hemisphere, which was divided inside in concentric circles, and used it as a portable instrument, named scaphion, for determination of geographical coordinates from measured solar altitudes. With this instrument Eratosthenes of Cyrene 220 B.C. had measured the length of Earth's meridian, and after that they used this instrument to survey smaller regions as well. Hipparchus had proposed to determine the geographical longitudes of several cities at solar eclipses. An eclipse does not occur simultaneously at all places on Earth, and their difference longitude can be computed from the difference in time when the eclipse is observed. His method would give the most accurate data as would any previous one, if it would be correctly carried out. But it was never properly applied, and for this reason maps remained rather inaccurate until modern times.
Ptolemy reported that Hipparchus invented an improved type of theodolite with which to measure angles.
We know that Hipparchus compiled one of the first catalogue of stars, and also compiled the first trigonometry tables. He tabulated values for the chord function, which gave the length of the chord for each angle. In modern terms, the chord of an angle equals twice the sine of half of the angle, e.g., chord(A) = 2 sin(A/2).
He had a method of solving spherical triangles[?]. The theorem in plane geometry called Ptolemy's theorem was developed by Hipparchus. This theorem was elaborated on by Carnot. Hipparchus was the first to show that the stereographic projection is conformal, and that it transforms circles on the sphere that do not pass through the center of projection to circles on the plane. This was the basis for the astrolabe.
Hipparchus is perhaps most famous for having been the first to measure the precession of the equinoxes. There is some suggestion that the Babylonians may have known about precession but it appears that Hipparchus was to first to really understand it and measure it. According to alBattani Chaldean astronomers had distinguished the tropical and siderical year. He stated they had around 330 B.C. an estimation for the length of the sidereal year to be S_{K} = 365^{d} 6^{h} 11^{m} (= 365.2576388^{d}) with an error of (about) 110^{s}. This phenomenon was probably also known to Kidinnu around 314 B.C.. A. Biot and Delambre attribute the discovery of precession also to old Chinese astronomers.
Hipparchus mostly used simple astronomical instruments such as the gnomon, astrolabe, armillary sphere and so. Before him Meton[?], Euktemon[?] and their students had determined 440 B.C. (431 B.C.) the two points of the solstice. Hipparchus on his own in Alexandria 146 B.C. determined the equinoctial point. He used Archimedes' observations of solstices. Hipparchus himself made several observations of the solstices and equinoxes. From these observations a year later in 145 B.C. he also on his own determined the length of the tropical year to be T_{H} = 365.24666...^{d} = 365^{d} 5^{h} 55 ^{m} 12 ^{s} (365^{d} + 1/4  1/300 = 365.24666...^{d} = 365^{d} 5^{h} 55 ^{m}), which differs from the actual value (modern estimate) T = 365.24219...^{d} = 365^{d} 5^{h} 48^{m} 45^{s} by only 6^{m} 27^{s} (6^{m} 15^{s}) (365.2423^{d} = 365^{d} 5^{h} 48^{m}, by only 7^{m}). We do not know the correct order for the precision of this value but most probably he was not able to make measurements within seconds so the correct value of his discovery was 365^{d} 5^{h} 55 ^{m}.
Before him the Chaldean astronomers knew the lengths of seasons are not equal. Hipparchus measured the full length of winter and spring to be 184 1/2 days, and of summer and autumn 180 1/2 days. In his geocentrical view, which he preferred, he explained this fact with the adoption that the Earth is not in the centre of Sun's orbit around it, but it lies eccentrically for 1/24 r. With his estimation of the length of seasons he tried to determine, as of today, the eccentricity of Earth's orbit, and according to Dreyer[?] he got the incorrect value e = 0.04166 (which is too large). The questions remains if he is really the author of this estimation.
After that from 141 B.C. to 126 B.C. mostly on the island of Rhodes, again in Alexandria and in Siracuse[?], and around 130 B.C. in Babylon, during which period he made a lot of precise and lasting observations. When he measured the length of gnomon shadow at solstice he determined the length of tropical year and he was finding times of the known bright star sunsets and times of sunrises. From all of these measurements he found in 134 B.C. the length of sidereal year to be S_{H} = 365^{d} 6^{h} 10^{m} (365.2569444...^{d}), which differs from today's S = 365.2563657...^{d} = 365^{d} 6^{h} 9^{m} 10^{s} for 50^{s}. Hipparchus also had measurements of the times of solstices from Aristarchus dating from 279 B.C. and from the school of Meton and Euctemon dating from 431 B.C.. This was a long enough period of time to allow him to calculate the difference between the length of the sidereal year and the tropical year, and led him to the discovery of precession. When he compared both lengths, he saw the tropical year is shorter for about 20 minutes from sidereal.
And as first in the history he correctly explained this with retrogradical movement of vernal point γ over the ecliptic for about 45", 46" or 47" (36" or 3/4' according to Ptolemy) per annum (today's value is Ψ'=50.387", 50.26") and he showed the Earth's axis is not fixed in space.
After that in 135 B.C., enthusiastic of a nova star in the constellation of Scorpius he measured with equatorial armillary sphere the ecliptical coordinates of about 850 (1600 or 1080, what is falsely quoted many times elsewhere) and till 129 B.C. he made first big star catalogue.
This map served him to find any changes on the sky and for great sadness it is not preserved today. His star map was thoroughly modificated as late as 1000 years later in 964 by A. Ali Sufi and 1500 years later in 1437 by Ulugh Beg. Later, Halley would use his star catalogue to discover proper motions as well. His work speaks for itself. And another sad fact is that we do not know almost nothing from his life, what was already stressed by Hoyle.
In his star map Hipparchus drew position of every star on the basis of its celestial latitude, (its angular distance from the celestial equator) and its celestial longitude (its angular distance from an arbitrary point, for instance as is custom in astronomy from vernal equinox). The system from his star map was also transferred to maps for Earth. Before him longitudes and latitudes were used by Dicaearchus of Messana, but they got their meanings in coordinate net not until Hipparchus.
By comparing his own measurements of the position of the equinoxes to the star Spica during a lunar eclipse at the time of equinox with those of Euclid's contemporaries Timocharis (circa 320 B.C.260 B.C.) of Alexandria and Aristyllus[?] 150 years earlier, the records of Chaldean astronomers and specially Kidinnu's records he still later observed that the equinox had moved 2° relative to Spica. He also noticed this motion in other stars. He obtained a value of not less than 1° in a century. The modern value is 1° in 72 years.
He also knew the works Phainomena (Phenomena) and Enoptron (Mirror of Nature) of Eudoxus of Cnidus, who had near Cyzicus on the southern coast of the Sea of Marmara his school and through Aratus' astronomical epic poem Phenomena Eudoxus' sphere, which was made from metal or stone and where there were marked constellations, brightest stars, tropic of Cancer and tropic of Capricorn. These comparisons embarrassed him because he couln't put together Eudoxus' detailed statements with his own observations and observations of that time. From all this he found that coordinates of the stars and the Sun had systematically changed. Their celestial latitudes λ ramained unchanged, but their celestial longitudes β had reduced as would equinoctial points, intersections of ecliptic and celestial equator, move with progressive velocity every year for 1/100'.
After him many Greek and Arab astronomers had confirmed this phenomenon. Ptolemy compared his catalogue with those of Aristyllus, Timocharis, Hipparchus and the observations of Agrippa and Menelaus of Alexandria from the early 1st century and he finally confirmed Hipparchus empirical fact that poles of celestial equator in one Platonic year or approximately in 25777 years encircle ecliptical pole[?]. The diameter of these cicles is equal to the inclination of ecliptic. The equinoctial points in this time traverse the whole ecliptic and they move for 1° in a century. This velocity is equal to Hipparchus' one. Because of these accordances Delambre, P. Tannery and other French historian of astronomy had wrongly jumped to conclusions that Ptolemy recorded his star catalogue from Hipparchus' one with an ordinary extrapolation. This was not known until 1898 when Marcel Boll and the others had found that Ptolemy's catalogue differs from Hipparchus' one not only in the number of stars but otherwise.
This phenomenon was named by Ptolemy just because the vernal point γ leads the Sun. In Latin praecesse means to overtake or to outpass and today means to twist or to turn too. Its own name shows this phenomenon was discovered practically before its theoretical explanation, otherwise would be named with a better term. Many later astronomers, physicists and mathematicians had occupied themselves with this problem, practically and theoretically. The phenomenon itself had opened many new promising solutions in several branches of celestial mechanics: Thabit's theory of trepidation and oscilation of equinoctial points, Newton's general gravitational law, which had explained it in full, Euler's kinematic equations and Lagrange's equations of motion, d'Alembert's dynamical theory of the movement of the rigid body, some algebraic solutions for special cases of precession, Flamsteed's and Bradley's difficulties in making of precise telescopic star catalogues, Bessel's and Newcomb's[?] measurements of precession and finally the precession of perihelion in Einstein's General Theory of Relativity.
Lunisolar precession[?] causes the motion of point γ by the ecliptic in the opposite direction od apparent solar year's movement and the circulation of celestial pole. This circle becomes a spiral because of additional ascendancy of the planets. This is planetary precession[?] where ecliptical plane swings from its central position for ±4° in 60000 years. The angle between ecliptic and celestial equator ε = 23° 26' is reducing for 0.47" per annum. Besides the point γ slides by equator for p = 0.108" per annum now in the same direction as the Sun. The sum of precessions gives an annual general precession in longitude Ψ = 50.288" which causes the origination of tropical year.
Hipparchus described the motion of the Sun and obtained a value for the eccentricity. It was known that the seasons were of unequal length, not something that would be expected if the Sun moved around the Earth in a circle at uniform speed (of course today we know that the planets move in ellipses, but this was not discovered until Kepler published his first two laws of planetary motion in 1609). His solution was to place the Earth not at the center of the Sun's motion, but at distance from the center. This model of the Sun's motion described the actual motion of the Sun fairly well.
Hipparchus also studied the motion of the Moon and obtained more accurate measurements of some periods of the motion than existed previously, and undertook to find the distances and sizes of the Sun and the Moon. He determined the length of synodic month to 23/50^{s} = 0.46^{s} about 139 B.C. in Babylon according to Strabo of Amaseia[?] in Pontus. He determined the Moon's horizontal parallax. He discovered the irregularity in lunar movement, which changes medium lunar longitude and today is called the equalization of the center with a value:
where m is medium anomaly[?] of the Moon.
Delambre in his Histoire de l'Astronomie Ancienne (1817) concluded that Hipparchus knew and used a real (celestial) equatorial coordinate system, directly with the right ascension and declination (or with its complement, polar distance). After that Otto Neugebauer[?] (18991990) in his A History of Ancient Mathematical Astronomy (1975) rejected Delambre's claims.
Hipparchus is believed to have died on the island of Rhodes.
An astrometric project of the Hipparcos Space Astrometry Mission of the European Space Agency (ESA) was named after him.
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