Hipparchus used to collect the records of the local weather conditions that were prevalent throughout the year when he was a young man living in Bithynia. The ‘weather calendars’ had been produced by various Greek astronomers since the 4th century BC which helped him understand the synchronization of the rains, winds, storms, seasons with the sun, moon and the constellations.
He began his scientific career in Bithynia and moved to Rhodes sometime before 141 BC.
He also worked in Alexandria for some time where he made some more observations. He probably had communications with observers in Alexandria and astronomers in Babylon who provided him with a lot of information on the time when equinoxes occurred.
He spent most of his adult life on the island of Rhodes to carry out various astronomical observations. According to Ptolemy, three observations were made from 162 to 158 BC and about twenty or more of these observations were made on specific dates ranging from 147 to 127 BC.
Out of these twenty or so observations made by Hipparchus, the first one was made on September 26 or 27, 147 BC on the autumnal equinox.
The last of these observations was taken of the lunar position on July 7, 127 BC.
His most important contribution to astronomy was the study of the orbits taken by the Sun and the Moon, determination of their sizes and their distances from the Earth. He assumed that the Earth was at the center of the Universe and all the other planets and stars, including the Sun that could be seen with the naked eye, revolved around it.
He devised a mathematical model with which the Sun’s orbital location from the Earth could be calculated on any date. This model was only elaborated by Johannes Kepler in the sixteenth century.
He tried to measure the length of a tropical year and succeeded in estimating the length to a period which was only longer by 6 minutes.
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Once he was able to calculate the time required for a tropical year to complete, he was easily able to calculate the equinox and solstice dates for any year. His theory on the precession of the equinoxes is considered accurate even today.
He also tried to create a theory on the eclipses that occur due to the movement of the Moon. He used the observations made by astronomers of Babylon of that time and confirmed that the motion of the Moon varies and diverges when it reaches the northern and southern part of its ecliptic orbit.
He is also reported to have measured the orbit taken by the Moon in relation to the Earth and calculated the distance from the Earth to the Moon to be seventy-seven times the radius of the Earth.
He also calculated the distance of the center of the Earth to the center of the Sun to be four-hundred and ninety times the radius of the Earth.
From the relation between the distances of the Sun and the Moon from the Earth, he calculated that the mean distance from the Earth to the Moon was sixty-three times the radius of the Earth which was accurately calculated later to be about sixty times the radius of the Earth.
Hipparchus is also said to have created a star catalog that had names and positions assigned to each star. But the number of stars in this catalog could not be ascertained as there is very little direct evidence on it. Ptolemy’s catalog consisting of 1,022 stars is presumed to be derived from Hipparchus’s original catalog.
His interest in geography was evident from the work he did in determining various terrestrial locations accurately.
He contributed greatly to the field of mathematics by inventing trigonometry which is based on a table consisting of the lengths of various chords in a circle of unit radius. He calculated the relative distances of various celestial bodies and their radius with the help of trigonometry.
The ‘European Space Agency’ named their ‘High Precision Parallax Collecting Satellite’ as ‘HiPParCoS’ after Hipparchus.
A Lunar crater has also been named after him as well as an asteroid called ‘4000 Hipparchus’.
His name also figures in the sixth position as one of the greatest astronomers in the world in the list brought out by an observatory in Los Angeles, California, USA.