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Hipparchus (Astronomer): Claudius Ptolemy
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Hipparchus is perhaps most famous for being almost universally recognized as discoverer of the precession of the equinoxes. 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 concluded that Spica had moved 2° relative to the autumnal equinox. He ... 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.
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Hipparchus was the most important Greek astronomers of his time. He very accurately cataloged over 1,000 stars and invented the mathematical science of trigonometry. Ptolemy was a great admirer of Hipparchus's research and recorded some of it in his book the "Almagest". Hipparchus studied the finding of earlier astronomers and compared them with his own. He discovered the procession of the equinoxes and was able to calculate the tropical year within 6.5 minutes of modern measurements. The tropical year is the length of the year measured by the Sun.
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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 ( AlmagestIII.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 ... have used another set of observations (94 + 1/4 and 92 + 3/4 days), which would lead to different values.
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Before Hipparchus, astronomers knew that the lengths of the seasons are not equal. Hipparchus made observations of equinox and solstice, and according to Ptolemy (Almagest III.4) determined that spring (from spring equinox to summer solstice) lasted 94½ days, and summer (from summer solstice to autumn equinox) 92½ days. This is inconsistent with 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 planets like the Earth move in ellipses 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 a little too large), and the direction of the apogee would be at longitude 65.5° from the vernal equinox.
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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.
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Swerdlow determined that Hipparchus relates the distances to the sun and moon using a construction found in Ptolemy. It would not be surprising if this calculation had been originally developed by Hipparchus himself, as he was a primary source for the Almagest.
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Claudius Ptolemy