Timocharis of Alexandria (Ancient Greek:Τιμόχαρις or Τιμοχάρης,gen. Τιμοχάρους; c. 320–260 BC) was aGreekastronomer andphilosopher. Likely born inAlexandria, he was a contemporary ofEuclid.
What little is known about Timocharis comes from citations byPtolemy in theAlmagest. These indicate that Timocharis worked in Alexandria during the 290s and 280s BC. Ptolemy lists thedeclination of 18 stars as recorded by Timocharis orAristillus in roughly the year 290 BC.[1] Between 295 and 272 BC, Timocharis recorded four lunaroccultations and the passage of the planetVenus across a star.[2] These were recorded using both theEgyptian andAthenian calendars.[3] The observed stellar passage by Venus may have occurred on October 12, 272 BC when the planet came within 15arcminutes of the starη Virginis.[4]
The observations by Timocharis are among the oldestGreek records that can be assigned a specific date. They are only exceeded by records of thesummer solstice of 432 BC, as noted byEuctemon andMeton.[5] Timocharis worked withAristillus in an astronomical observatory that was most likely part of theLibrary of Alexandria. Their equipment would have been simple, most likely consisting ofgnomons,sundials and anarmillary sphere. The two were contemporaries ofAristarchus of Samos, but it is unclear whether there was any association between Timocharis and Aristarchus.[6]
During his astronomical observations, Timocharis recorded that the starSpica was located 8° west of theAutumnal equinox. Later,Hipparchus observed that Spica was only 6° west of the Autumnal equinox. Hipparchus was able to deduce the period during which Timocharis made his observations based upon the records of earlier lunar eclipses. From this difference, Hipparchus discovered that thelongitudes of the stars had changed over time, which led him to determine the first value of theprecession of the equinoxes as no less than 1/100° per year.[5]
In approximately 3rd century BC, with the help ofAristillus, he created the firststar catalogue in the Western world.[citation needed]
The craterTimocharis on theMoon is named after him.[7]
Galileo assigns to Arsatilis[8] and Timocharis the origin "third opinion" on the number of heavens, namely that there were nine heavens, seven for the planets, one for the daily movement of the firmament from east to west, and another for the slower motion from west to east, while crediting Hipparchus,Agrias,Milaeus andPtolemy with improving the model.[9] This was when Galileo was of the opinion that there were ten movable heavens, and an eleventh immovable.[10]
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