MATHEMATICIAN, ASTRONOMER, GEOGRAPHER

Life
The greatest astronomical observer of all time, and the father of astronomy, Hipparchus was born in Nicaea in Bithynia, but lived most of his life in Rhodes and Alexandria. He insisted that geography - the determination of the positions of places on earth - had to be based on the use of astronomical methods to determine their latitude and longitude: he determined latitude by application of the gnomon, the zenith of the fixed stars, the duration of the longest day of the year, and thought that longitude could be determined by observing, from each place, the moments when a solar eclipse began and ended.


Work
His maps were based on geometric calculation, and marked a great step forward in the history of cartography. One of the craters on the moon has been named "Hipparchus" in his honour. His work is chronicled in Stobaeus.

Hipparchus invented an improved type of astrolabe, which he used to determine with accuracy the co-ordinates of the stars. He devised a globe and a planisphere. The planisphere allowed "stereographic projections" - also invented by Hipparchus - to be used to determine the precise time. He improved the dioptra, an instrument used to estimate the apparent diameter, distance and size of the sun and the moon, and used and improved various older instruments, including the plumb, the gnomon, the polos, the sundial, the clepsydra, the fixed sphere, the water clock and the rings.

Hipparchus was the first to divide the circle into 360?. He recognised that the earth is round, and constructed the first globe. In 134 BC he observed a new star (probably a comet) in the constellation of Scorpio, and formulated the astronomical principle that "the stars are not eternally fixed in the heavens". He calculated the length of the tropical year at 365.24667 days (the true value is 365.242217) and the "inclination of the ecliptic" (the angle of the earth's orbit to the equator) at 23? 51´ (true value: 23? 43´ at the time of Hipparchus). He calculated, on the basis of observations of eclipses, that the average distance of the moon from the earth is 33.66 times the diameter of the earth (true value: 30.20) and that the diameter of the moon is 1/3 that of the earth (true value" 0.27). He calculated the dates of eclipses of the moon. He calculated the length of the great circle (equator) of the earth at 39,960 kilometres (true value: 40,000 km), that is, with an error of just 40 km, or 1/1000! He applied mathematical principles to the determination of places on the earth's surface, by specifying their latitude and longitude. Longitude he thought could be determined by observing the time of eclipses at the various places. He observed the planets and their orbits, founded plane and spherical trigonometry, and drew up a table of chords (sines). He was the first to apply the "stereographic projection of the sphere", that is, the representation of the surface of a sphere as a plane, still used today in the preparation of geographical maps. He wrote a criticism of the work of Eratosthenes.

Principal work:

"Catalogue of stars": Prepared in 127 BC. Extant. Lists the 1039 most brilliant of the then visible stars, with their "celestial latitude and longitude", that is, their celestial co-ordinates.

Other books:

"On constellations"

"On the arrangement of the fixed stars"

"On the treatise on simultaneous risings"

"On the rising of the 12 constellations of the zodiac"

"On the precession of the equinoxes"

"On the parallax" (2 books)

"On the size and distance of the sun and moon"

"On the inequality of the motion of the moon"

"On the eclipses of the sun in the seven climates"

"On the lunar year"

"On the intercalary months and days"

"On the length of the year"

"Investigation of the chords of a circle" (12 books)

"On gravity"

"Against the Geography of Eratosthenes"

"To the noblest"

"On the Phaenomena of Eudoxus and Aratus" (2 books)

This last work is extant. The others were all lost when the Library at Alexandria was burned down. Fortunately, extensive extracts from these works and references to them are preserved in other writers, including Ptolemy, Pliny, Strabo, Theon of Smyrna, Theon of Alexandria and Plutarch.