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HIPPARCHUS (c 190 – c120 BC)

Hipparchus was a Greek astronomer, mathematician and geographer. Surprisingly little is known about his early life. He was most certainly born in Nicaea, Bithynia, (which is now known as Iznik in Turkey), around 190 BC. Most of what we do about him comes from the books of other scholars who came after him, such as Ptolemy and Strabo. It seems likely that Hipparchus studied in Alexandria but spent his later life in Rhodes. Strabo, another Greek geographer writing about eighty years after Hipparchus’ death, describes him as “one of the famous men of Bithynia”. The astronomer Claudius Ptolemy, who extended some of Hipparchus’ work about two hundred and fifty years later, admired his hard work and his quest for truth.

Hipparchus is recorded has being the first to make a catalogue of stars. He probably made his observations while living in Rhodes, from about 146 BC. He observed 850 stars and graded them according to their brightness. His brightness scale ranged from 1 (the brightest) to 6 (the dimmest). Interestingly enough, Hipparchus scale is still used today! The only difference is that he did not have the luxury of using a telescope. He could only see what we now call 6th magnitude stars (those that are just visible to the naked eye). From an observatory today we can see stars of the 22nd magnitude. Hipparchus also listed the stars in each constellation and noted their positions. Ptolemy continued his work and catalogued another 37 stars, bringing the total to 1080.

Aratus Hipparchus

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As a mathematician, Hipparchus is credited with dividing a circle into 360° and being the first to use trigonometry. Using observations during the solar eclipse, probably in129 BC, he was able to work out the distance from the Earth to the Moon. The eclipse at Alexandria had been partial, four fifths of the sun had been covered, but at Syene it had been total. By measuring angles and applying distances between the two cities he was able to complete his calculations. Hipparchus also used mathematics to allow him to predict equinoxes, to accurately measure the length of the seasons and to calculate the size of the Moon and the Sun.

Much of Hipparchus’ work was only possible because he was able to consult the work of astronomers who had gone before. The ancient Babylonians had left records of their astronomical observations, methods and equipment. There is much evidence to show that Hipparchus used this information and that allowed him to make comparisons. Hipparchus’ precise measurement of the equinoxes was, in fact, a mathematical interpretation of ancient Babylonian knowledge. He also referred to the observations of Timocharis, an astronomer who had lived 150 years before Hipparchus, to be able to predict equinoxes.

Hipparchus made the first star chart which showed the position and brightness of stars on a globe. He is credited with the invention the astrolabe around 150 BC. This instrument can measure the movement of the sun and stars. The user can determine the time at night from the positions of the stars and can find the time of the sunrise and sunset. The astrolabe was introduced to Europe by Arab astronomers in the 10th Century. It was still in use, particular by sailors, up until the 19th Century.

That Hipparchus’ work is thought highly of is shown by the honours that he received, and continues to receive, since his death. Coins have been found from Nicaea that show him looking at his star chart, he appears on Roman coins from five different emperors reigns (138 AD and 253 AD). He has appeared on the covers of many astronomy papers and books. In more recent years he has had a Moon crater named after him, a crater on Mars, an asteroid and an ESA satellite!

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