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Hipparchus ( Greek ) (ca. 190 BC – ca. 120 BC ) was a Hellenistic Astronomer , Geographer , and Mathematician . Hipparchus was born in Nicaea (now has been preserved by later copyists. As a consequence, we know comparatively little about Hipparchus. LIFE AND WORK Most of what is known about Hipparchus comes from Ptolemy 's ( 2nd Century ) '' Almagest '' ("the great treatise"; ed. 1981 ), with additional references to him by Pappus Of Alexandria and Theon Of Alexandria ( 4th Century ) in their commentaries on the ''Almagest''; from Strabo 's ''Geographia'' ("Geography"), and from Pliny The Elder 's '' Naturalis Historia '' ("Natural history") ( 1st Century ). There is a strong tradition that Hipparchus was born in Nicaea (Greek ''Νικαία''), in the ancient district of Bithynia (modern-day Iznik in province Bursa ), in what today is Turkey . The exact dates of his life are not known, but Ptolemy attributes astronomical observations to him from 147 BC to 127 BC ; earlier observations since 162 BC might also be made by him. The date of his birth (ca. 190 BC ) was calculated by Delambre based on clues in his work. Hipparchus must have lived some time after 127 BC because he analyzed and published his latest observations. Hipparchus obtained information from Alexandria as well as Babylon , but it is not known if and when he visited these places. It is not known what Hipparchus' economic means were and how he supported his scientific activities. Also, his appearance is unknown: there are no contemporary portraits.In the 2nd and 3rd centuries Coin s were made in his honour in Bithynia that bear his name and show him with a Globe ; this confirms the tradition that he was born there. Hipparchus is believed to have died on the island of Rhodes , where he spent most of his later life — Ptolemy attributes observations to him from Rhodes in the period from 141 BC to 127 BC . Hipparchus's main original works are lost. His only preserved work is ''Toon Aratou kai Eudoxou Fainomenoon exegesis'' ("Commentary on the Phaenomena of Eudoxus and Aratus"). This is a critical commentary in two books on a popular 2005 . Hipparchus is recognized as the originator and father of scientific Astronomy . He is believed to be the greatest Hellenistic astronomical observer, and many regard him as the greatest astronomer of ancient times, although Cicero gave preferences to Aristarchus Of Samos . Some put in this place also Ptolemy of Alexandria. Hipparchus' writings had been mostly superseded by those of Ptolemy, so later copyists have not preserved them for posterity. There is evidence, based on references in non-scientific writers such as Plutarch, that Hipparchus was aware of some physical ideas that we consider Newtonian, and that Newton knew this. (See Lucio Russo, ''The Forgotten Revolution'', Springer, 2004; Italian edition, 1996.) Also see the biographical articles by 1978 and 2001 . BABYLONIAN SOURCES ''See also'': Babylonian Influence On Greek Astronomy Many of the works of Greek scientists - mathematicians, astronomers, geographers - have been preserved up to the present time, or some aspects of their work and thought are still known through later references. However, achievements in these fields by Ancient Near East ern civilizations, notably those in Babylonia , had been forgotten. After the discovery of the archaeological sites in the 19th Century , many writings on clay tablets have been found, some of them related to astronomy. Most known astronomical tablets have been described by A. Sachs , and later published by Otto Neugebauer in "Astronomical Cuneiform Texts" (3 vol.; Princeton and London, 1955). Since the rediscovery of the Babylonian civilization, it has become apparent that Greek and Hellenistic astronomers, and in particular Hipparchus, borrowed a lot from the Chaldea ns. , specifically the collection of texts nowadays called "System B" (sometimes attributed to Kidinnu ). Apparently Hipparchus only confirmed the validity of the periods he learned from the Chaldeans by his newer observations. Other traces of Babylonian practice in Hipparchus' work are:
Also see G.J. Toomer (1981?): ''Hipparchus and Babylonian Astronomy''. GEOMETRY AND TRIGONOMETRY Hipparchus is recognised as the first mathematician who compiled a of half of the angle, i.e.: : chord(''A'') = 2 sin(''A''/2). He described it in a work (now lost), called ''Toon en kuklooi eutheioon'' (''Of Lines Inside a Circle'') by Theon Of Alexandria ( 4th Century ) in his commentary on the ''Almagest'' I.10; some claim his table may have survived in astronomical treatises in India , for instance the '' Surya Siddhanta ''. This was a significant innovation, because it allowed Greek astronomers to solve any triangle, and made it possible to make quantitative astronomical models and predictions using their preferred geometric techniques. See 1973 . For his chord table Hipparchus must have used a better approximation for ) (''Almagest'' VI.7); but it is not known if he computed an improved value himself. Hipparchus could construct his chord table using the Pythagorean Theorem and a Theorem known to Archimedes. He also might have developed and used the theorem in Plane Geometry called Ptolemy's Theorem , because it was proved by Ptolemy in his ''Almagest'' (I.10) (later 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 . Besides geometry, Hipparchus also used Arithmetic techniques from the Chaldea ns. He was one of the first Greek mathematicians to do this, and in this way expanded the techniques available to astronomers and geographers. There is no indication that Hipparchus knew Spherical Trigonometry , which was first developed by Menelaus Of Alexandria in the 1st Century . Ptolemy later used the new technique for computing things like the rising and setting points of the Ecliptic , or to take account of the lunar Parallax . Hipparchus may have used a globe for this (to read values off the coordinate grids drawn on it), as well as approximations from planar geometry, or arithmetical approximations developed by the Chaldeans. LUNAR AND SOLAR THEORY Motion of the Moon Hipparchus also studied the motion of the Moon and confirmed the accurate values for some periods of its motion that Chaldean astronomers had obtained before him. The traditional value (from Babylonian System B) for the mean Synodic Month is 29 days;31,50,8,20 (sexagesimal) = 29.5305941... d. Expressed as 29 days + 12 hours + 793/1080 hours this value has been used later in the Hebrew Calendar (possibly from Babylonian sources). The Chaldeans also knew that 251 Synodic Month s = 269 Anomalistic Month s. Hipparchus extended this period by a factor of 17, because after that interval the Moon also would have a similar latitude, and it is close to an integer number of years (345). Therefore, eclipses would reappear under almost identical circumstances. The period is 126007 days 1 hour (rounded). Hipparchus could confirm his computations by comparing eclipses from his own time (presumably 27 January 141 BC and 26 November 139 BC according to 1980 ), with eclipses from Babylonian records 345 years earlier (''Almagest'' IV.2; 2001 ). Already Al-Biruni (''Qanun'' VII.2.II) and Copernicus (''de revolutionibus'' IV.4) noted that the period of 4,267 lunations is actually about 5 minutes longer than the value for the eclipse period that Ptolemy attributes to Hipparchus. However, the best clocks and timing methods of the age had an accuracy of no better than 8 minutes. Modern scholars agree that Hipparchus rounded the eclipse period to the nearest hour, and used it to confirm the validity of the traditional values, rather than try to derive an improved value from his own observations. From modern ephemerides ''et al.'' 2002 and taking account of the change in the length of the day (see ΔT ) we estimate that the error in the assumed length of the synodic month was less than 0.2 s in the 4th Century BC and less than 0.1 s in Hipparchus' time. Orbit of the Moon It had been known for a long time that the motion of the Moon is not uniform: its speed varies. This is called its anomaly, and it repeats with its own period; the Anomalistic Month . The Chaldeans took account of this arithmetically, and used a table giving the daily motion of the Moon according to the date within a long period. The Greeks however preferred to think in geometrical models of the sky. Apollonius Of Perga had at the end of the 3rd Century BC proposed two models for lunar and planetary motion: # In the first, the Moon would move uniformly along a circle, but the Earth would be eccentric, i.e., at some distance of the center of the circle. So the apparent angular speed of the Moon (and its distance) would vary. # The Moon itself would move uniformly (with some mean motion in anomaly) on a secondary circular orbit, called an epicycle, that itself would move uniformly (with some mean motion in longitude) over the main circular orbit around the Earth, called '''deferent'''; see Deferent And Epicycle . Apollonius demonstrated that these two models were in fact mathematically equivalent. However, all this was theory and had not been put to practice. Hipparchus was the first to attempt to determine the relative proportions and actual sizes of these Orbit s. Hipparchus devised a geometrical method to find the parameters from three positions of the Moon, at particular phases of its anomaly. In fact, he did this separately for the eccentric and the epicycle model. Ptolemy describes the details in the ''Almagest'' IV.11. Hipparchus used two sets of three lunar eclipse observations, which he carefully selected to satisfy the requirements. The eccentric model he fitted to these eclipses from his Babylonian eclipse list: 22/23 December 383 BC , 18/19 June 382 BC , and 12/13 December 382 BC . The epicycle model he fitted to lunar eclipse observations made in Alexandria at 22 September 201 BC , 19 March 200 BC , and 11 September 200 BC .
The somewhat weird numbers are due to the cumbersome unit he used in his chord table. The results are distinctly different. This is partly due to some sloppy rounding and calculation errors, for which Ptolemy criticised him (he himself made rounding errors too). Anyway, Hipparchus found inconsistent results; he later used the ratio of the epicycle model (3122+1/2 : 247+1/2), which is too small (60 : 4;45 hexadecimal): Ptolemy established a ratio of 60 : 5+1/4 . See 1967 . Apparent motion of the Sun Before Hipparchus, Meton , Euctemon , and their pupils at Athens had made a solstice observation (i.e., timed the moment of the summer Solstice ) on June 27 , 432 BC ( Proleptic Julian Calendar ). Aristarchus Of Samos is said to have done so in 280 BC , and Hipparchus also had an observation by Archimedes . Hipparchus himself observed the summer solstice in 135 BC , but he found observations of the moment of Equinox more accurate, and he made many during his lifetime. Ptolemy gives an extensive discussion of Hipparchus' work on the length of the year in the ''Almagest'' III.1, and quotes many observations that Hipparchus made or used, spanning 162 BC to 128 BC . Ptolemy quotes an equinox timing by Hipparchus (at is too high, which changes the observed time when the Sun crosses the equator. Worse, the refraction decreases as the Sun rises, so it may appear to move in the wrong direction with respect to the equator in the course of the day - as Ptolemy mentions; however, Ptolemy and Hipparchus apparently did not realize that refraction is the cause. At the end of his career, Hipparchus wrote a book called ''Peri eniausíou megéthous'' ("On the Length of the Year") about his results. The established value for the Tropical Year , introduced by Callippus in or before 330 BC (possibly from Babylonian sources, see above), was 365 + 1/4 days. Hipparchus' equinox observations gave varying results, but he himself points out (quoted in ''Almagest'' III.1(H195)) that the observation errors by himself and his predecessors may have been as large as 1/4 day. So he used the old solstice observations, and determined a difference of about one day in about 300 years. So he set the length of the tropical year to 365 + 1/4 - 1/300 days (= 365.24666... days = 365 days 5 hours 55 min, which differs from the actual value (modern estimate) of 365.24219... days = 365 days 5 hours 48 min 45 s by only about 6 min). Between the solstice observation of Meton and his own, there were 297 years spanning 108,478 days. This implies a tropical year of 365.24579... days = 365 days;14,44,51 (sexagesimal; = 365 days + 14/60 + 44/602 + 51/603), and this value has been found on a Babylonian clay tablet Jones, 2001 . This is an indication that Hipparchus' work was known to Chaldeans. Another value for the year that is attributed to Hipparchus (by the astrologer (actual value at his time (modern estimate) ca. 365.2565 days), but the difference with Hipparchus' value for the tropical year is consistent with his rate of Precession (see below). Orbit of the Sun Before Hipparchus the Chaldean astronomers knew that the lengths of the Season s are not equal. Hipparchus made equinox and solstice observations, and according to Ptolemy (''Almagest'' III.4) determined that spring (from spring equinox to summer solstice) lasted 94 + 1/2 days, and summer (from summer solstice to autumn equinox) 92 + 1/2 days. This is an unexpected result given a premise of the Sun moving around the Earth in a circle at uniform speed. Hipparchus' solution was to place the Earth not at the center of the Sun's motion, but at some distance from the center. This model described the apparent motion of the Sun fairly well (of course today we know that the Planet s like the Earth move in Ellipse s around the Sun, but this was not discovered until Johannes Kepler published his first two laws of planetary motion in 1609 ). The value for the Eccentricity attributed to Hipparchus by Ptolemy is that the offset is 1/24 of the radius of the orbit (which is too large), and the direction of the Apogee would be at longitude 65.5° from the Vernal Equinox . Hipparchus may also have used another set of observations (94 + 1/4 and 92 + 3/4 days), which would lead to different values. The question remains if Hipparchus is really the author of the values provided by Ptolemy, who found no change three centuries later, and added lengths for the autumn and winter seasons. Distance, parallax, size of the Moon and Sun See Also: Hipparchus On Sizes and Distances Hipparchus also undertook to find the distances and sizes of the Sun and the Moon. He published his results in a work of two books called ''Peri megethoon kai 'apostèmátoon'' ("On Sizes and Distances") by Pappus in his commentary on the ''Almagest'' V.11; Theon Of Smyrna ( 2nd Century ) mentions the work with the addition "of the Sun and Moon". Hipparchus measured the apparent diameters of the Sun and Moon with his ''diopter''. Like others before and after him, he found that the Moon's size varies as it moves on its (eccentric) orbit, but he found no perceptible variation in the apparent diameter of the Sun. He found that at the Mean distance of the Moon, the Sun and Moon had the same apparent diameter; at that distance, the Moon's diameter fits 650 times into the circle, i.e., the mean apparent diameters are 360/650 = 0°33'14". Like others before and after him, he also noticed that the Moon has a noticeable Parallax , i.e., that it appears displaced from its calculated position (compared to the Sun or Star s), and the difference is greater when closer to the horizon. He knew that this is because the Moon circles the center of the Earth, but the observer is at the surface - Moon, Earth and observer form a triangle with a sharp angle that changes all the time. From the size of this parallax, the distance of the Moon as measured in Earth Radii can be determined. For the Sun however, there was no observable parallax (we now know that it is about 8.8", more than ten times smaller than the resolution of the unaided eye). In the first book, Hipparchus assumes that the parallax of the Sun is 0, as if it is at infinite distance. He then analyzed a solar eclipse, presumably that of 14 March 190 BC . It was total in the region of the Hellespont (and in fact in his birth place Nicaea); at the time the Romans were preparing for war with Antiochus III in the area, and the eclipse is mentioned by Livy in his '' Ab Urbe Condita '' VIII.2. It was also observed in Alexandria, where the Sun was reported to be obscured for 4/5 by the Moon. Alexandria and Nicaea are on the same meridian. Alexandria is at about 31° North, and the region of the Hellespont at about 41° North; authors like Strabo and Ptolemy had fairly decent values for these geographical positions, and presumably Hipparchus knew them too. So Hipparchus could draw a triangle formed by the two places and the Moon, and from simple geometry was able to establish a distance of the Moon, expressed in Earth radii. Because the eclipse occurred in the morning, the Moon was not in the Meridian , and as a consequence the distance found by Hipparchus was a lower limit. In any case, according to Pappus, Hipparchus found that the least distance is 71 (from this eclipse), and the greatest 81 Earth radii. In the second book, Hipparchus starts from the opposite extreme assumption: he assigns a (minimum) distance to the Sun of 470 Earth radii. This would correspond to a parallax of 7', which is apparently the greatest parallax that Hipparchus thought would not be noticed (for comparison: the typical resolution of the human eye is about 2'; Tycho Brahe made naked eye observation with an accuracy down to 1'). In this case, the shadow of the Earth is a Cone rather than a Cylinder as under the first assumption. Hipparchus observed (at lunar eclipses) that at the mean distance of the Moon, the diameter of the shadow cone is 2+½ lunar diameters. That apparent diameter is, as he had observed, 360/650 degrees. With these values and simple geometry, Hipparchus could determine the mean distance; because it was computed for a minumum distance of the Sun, it is the maximum mean distance possible for the Moon. With his value for the eccentricity of the orbit, he could compute the least and greatest distances of the Moon too. According to Pappus, he found a least distance of 62, a mean of 67+1/3, and consequently a greatest distance of 72+2/3 Earth radii. With this method, as the parallax of the Sun decreases (i.e., its distance increases), the minimum limit for the mean distance is 59 Earth radii - exactly the mean distance that Ptolemy later derived. Hipparchus thus had the problematic result that his minimum distance (from book 1) was greater than his maximum mean distance (from book 2). He was intellectually honest about this discrepancy, and probably realized that especially the first method is very sensitive to the accuracy of the observations and parameters (in fact, modern calculations show that the size of the solar eclipse at Alexandria must have been closer to 9/10 than to the reported 4/5). Ptolemy later measured the lunar parallax directly (''Almagest'' V.13), and used the second method of Hipparchus' with lunar eclipses to compute the distance of the Sun (''Almagest'' V.15). He criticizes Hipparchus for making contradictory assumptions, and obtaining conflicting results (''Almagest'' V.11): but apparently he failed to understand Hipparchus' strategy to establish limits consistent with the observations, rather than a single value for the distance. His results were the best so far: the actual mean distance of the Moon is 60.3 Earth radii, within his limits from book 2. Theon Of Smyrna wrote that according to Hipparchus, the Sun is 1,880 times the size of the Earth, and the Earth twenty-seven times the size of the Moon; apparently this refers to Volume s, not Diameter s. From the geometry of book 2 it follows that the Sun is at 2,550 Earth radii, and the mean distance of the Moon is 60½ radii. Similarly, Cleomedes quotes Hipparchus for the sizes of the Sun and Earth as 1050:1; this leads to a mean lunar distance of 61 radii. Apparently Hipparchus later refined his computations, and derived accurate single values that he could use for predictions of solar eclipses. See 1974 for a more detailed discussion. Eclipses Pliny (''Naturalis Historia'' II.X) tells us that Hipparchus demonstrated that lunar eclipses can occur five months apart, and solar eclipses seven months (instead of the usual six months); and the Sun can be hidden twice in thirty days, but as seen by different nations. Ptolemy discussed this a century later at length in ''Almagest'' VI.6. The geometry, and the limits of the positions of Sun and Moon when a solar or lunar eclipse is possible, are explained in ''Almagest'' VI.5. Hipparchus apparently made similar calculations. The result that two solar eclipses can occur one month apart is important, because this can not be based on observations: one is visible on the northern and the other on the southern hemisphere - as Pliny indicates -, and the latter was inaccessible to the Greek. Prediction of a solar eclipse, i.e., exactly when and where it will be visible, requires a solid lunar theory and proper treatment of the lunar parallax. Hipparchus must have been the first to be able to do this. A rigorous treatment requires Spherical Trigonometry , but Hipparchus may have made do with planar approximations. He may have discussed these things in ''Peri tes kata platos meniaias tes selenes kineseoos'' ("On the monthly motion of the Moon in latitude"), a work mentioned in the '' Suda ''. Pliny also remarks that "he also discovered for what exact reason, although the shadow causing the eclipse must from sunrise onward be below the earth, it happened once in the past that the moon was eclipsed in the west while both luminaries were visible above the earth." (translation H. Rackham (1938), to the Sun. Parallax lowers the altitude of the luminaries; refraction raises them, and from a high point of view the horizon is lowered. ASTRONOMICAL INSTRUMENTS AND ASTROMETRY Hipparchus and his predecessors mostly used simple instruments for astronomical calculations and observations, such as the Gnomon , the Astrolabe , and the Armillary Sphere . Hipparchus is credited with the invention or improvement of several astronomical instruments, which were used for a long time for (which Ptolemy however says he constructed, in ''Almagest'' V.1); or the predecessor of the planar instrument called Astrolabe (also mentioned by Theon Of Alexandria ). With an astrolabe Hipparchus was the first to be 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 '', or with the portable instrument known as '' Scaphion ''. .]] Ptolemy mentions (''Almagest'' V.14) that he used a similar instrument as Hipparchus, called '' Dioptra '', to measure the apparent diameter of the Sun and Moon. Pappus Of Alexandria described it (in his commentary on the ''Almagest'' of that chapter), as did Proclus (''Hypotyposis'' IV). It was a 4-foot rod with a scale, a sighting hole at one end, and a wedge that could be moved along the rod to exactly obscure the disk of Sun or Moon. Hipparchus also observed solar (i.e., in one of the equinoctial points on the Ecliptic ), but the shadow falls above or below the opposite side of the ring when the Sun is south or north of the equator. Ptolemy quotes (in ''Almagest'' III.1 (H195)) a description by Hipparchus of an equatorial ring in Alexandria; a little further he describes two such instruments present in Alexandria in his own time. GEOGRAPHY Hipparchus applied his knowledge of spherical angles to the problem of denoting locations on the Earth's surface. Before him a grid system had been used by Dicaearchus of Messana , but Hipparchus was the first to apply mathematical rigor to the determination of the Latitude and Longitude of places on the Earth. Hipparchus wrote a critique in three books on the work of the geographer Eratosthenes of Cyrene ( 3rd Century BC ), called ''Pròs tèn 'Eratosthénous geografían'' ("Against the Geography of Eratosthenes"). It is known to us from Strabo of Amaseia, who in his turn criticised Hipparchus in his own ''Geografia''. Hipparchus apparently made many detailed corrections to the locations and distances mentioned by Eratosthenes. It seems he did not introduce many improvements in methods, but he did propose a means to determine the Geographical Longitudes of different Cities at Lunar Eclipse s (Strabo ''Geografia'' 7). A lunar eclipse is visible simultaneously on half of the Earth, and the difference in longitude between places can be computed from the difference in local time when the eclipse is observed. His approach would give accurate results if it were correctly carried out but the limitations of timekeeping accuracy in his era made this method impractical. STAR CATALOGUE After that, in 135 BC , enthusiastic about a Nova in the constellation of Scorpius , he measured with an Equatorial Armillary Sphere Ecliptical Coordinates of about 1,000 stars (the exact number is not known) for his Star Catalogue . He also knew the work ''Phainomena'' (''Phenomena''). That poem, known as ''Phaenomena'' or ''Arateia'', describes the Constellation s and the Star s that form them. Hipparchus' commentary contains many measurements of Stellar Position and times for rising, culmination, and setting of the constellations treated in the ''Phaenomena'', and these are likely to have been based on measurements of Stellar Position s— and he knew the ''Enoptron'' (''Mirror of Nature'') of Eudoxus Of Cnidus , who had his school near Cyzicus on the southern coast of the Sea Of Marmara and through the ''Phenomena'' Eudoxus' sphere, which was made from metal or stone and where there were marked constellations, brightest stars, the Tropic Of Cancer and the Tropic Of Capricorn . These comparisons embarrassed him because he could not 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 ecliptic latitudes β remained unchanged, but their ecliptic longitudes λ had increased, at a rate which he estimated to be at least one degree per century. This catalog served him to find any changes on the sky but unfortunately it is not preserved today. However, a by Al Sufi and 1,500 years later ( 1437 ) by Ulugh Beg . Later, Halley would use his star catalogue to discover proper motions as well. The system of celestial coordinates used in Hipparchus's star catalog is not known. Since Ptolemy's copy in the Almagest is given in Ecliptical Coordinates , that system would seem the most likely; although there is evidence that both ecliptic coordinates and Equatorial Coordinates were used in the original observations. Celestial Bodies Hipparchus in 130 BC wrote about an Open Cluster , the M44 Praesepe ( NGC 2632 ) as a "Little Cloud " or "Cloudy Star". Before him the object was known to Aratus ca. 260 BC , who wrote about it as a "Little Mist ". Hipparchus also included this object in his famous star catalogue. The cluster was also known to Chinese astronomers. 1994 , [10] Celestial Coordinate System s 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 ). Later Otto Neugebauer in his ''A History of Ancient Mathematical Astronomy'' ( 1975 ) rejected Delambre's claims. Brightness of stars Hipparchus had in and a similar system is still in use today. (See Apparent Magnitude ). PRECESSION OF THE EQUINOXES ( 146 BC - 130 BC ) ''See also'': Discovery Of Precession Hipparchus is perhaps most famous for having discovered the Precession of the Equinox es. His two books on precession, ''On the Displacement of the Solsticial and Equinoctial Points'' and ''On the Length of the Year'', are both mentioned in the '' Almagest '' of Claudius Ptolemy . According to Ptolemy, Hipparchus measured the longitude of Spica and other bright stars. Comparing his measurements with data from his predecessors, Timocharis and Aristillus , he realized that Spica had moved 2° relative to the Autumnal Equinox . He also compared the lengths of the Tropical Year (the time it takes the Sun to return to an equinox) and the Sidereal year (the time it takes the Sun to return to a fixed star), and found a slight discrepancy. Hipparchus concluded that the equinoxes were moving ("precessing") through the zodiac, and that the rate of precession was not less than 1° in a century. Ptolemy followed up on Hipparchus' work in the 2nd century CE. He confirmed that precession affected the entire sphere of fixed stars (Hipparchus had speculated that only the stars near the zodiac were affected), and concluded that 1° in 100 years was the correct rate of precession. The modern value is 1° in 72 years. HIPPARCHUS AND ASTROLOGY In addition to his other writings dealing with Astronomical topics, the work of Hipparchus dealing with the calculation and prediction of celestial positions would have been very useful to those engaged in Astrology . Astrology developed in the Greco-Roman world during the Hellenistic period, borrowing many elements from Babylonian astronomy; some historians have suggested that Hipparchus played a key role in this. Remarks made by Pliny The Elder (who died 79 AD during the eruption of the Volcano Mount Vesuvius ), in his ''Natural History'' Book 2.24, suggest that some Ancient authors did regard Hipparchus as an important figure in the History Of Astrology . Pliny claimed that Hipparchus "can never be sufficiently praised, no one having done more to prove that man is related to the stars and that our souls are a part of heaven." NAMED AFTER HIPPARCHUS The ESA 's Hipparcos Space Astrometry Mission was named after him, as are the Hipparchus Lunar Crater and the Asteroid 4000 Hipparchus . SEE ALSO
LITERATURE
EXTERNAL LINKS General
Precession
Celestial bodies
Star catalogue
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