What little is known about Timocharis comes from citations by Ptolemy in the Almagest. These indicate that Timocharis worked in Alexandria during the 290s and 280s BCE. Ptolemy lists the declination of 18 stars as recorded by Timocharis or Aristillus in roughly the year 290 BCE. Between 295 and 272 BCE, Timocharis recorded four lunar occultations and the passage of the planet Venus across a star. These were recorded using both the Egyptian and Athenian calendars. The observed stellar passage by Venus may have occurred on October 12, 272 BCE when the planet came within 15 arcminutes of the star η Virginis.
The observations by Timocharis are among the oldest Greek records that can be assigned a specific date. They are only exceeded by records of the summer solstice of 432 BCE, as noted by Euctemon and Meton. Timocharis worked with Aristillus in an astronomical observatory that was most likely part of the Library of Alexandria. Their equipment would have been simple, most likely consisting of gnomons, sundials and an armillary sphere. The two were contemporaries of Aristarchus of Samos, but it is unclear whether there was any association between Timocharis and Aristarchus.
During his astronomical observations, Timocharis recorded that the star Spica was located 8° west of the Autumnal 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 the longitudes of the stars had changed over time, which led him to determine the first value of the precession of the equinoxes as no less than 1/100° per year.
He made the third recorded mention of Mercury.
- Newton, R. R. (1974). "The obliquity of the ecliptic two millenia ago". Monthly Notices of the Royal Astronomical Society 169: 331–342. Bibcode:1974MNRAS.169..331N. doi:10.1093/mnras/169.2.331.
- Jones, Alexander (1999). Astronomical papyri from Oxyrhynchus 1–2. DIANE Publishing. p. 84. ISBN 0-87169-233-3.
- Jones, A. (1997), On the reconstructed Macedonian and Egyptian lunar calendars (PDF) 119, pp. 157–166, retrieved 2009-09-10
- Fomenko, A. T.; Vi︠a︡cheslavovich, Vladimir Kalashnikov; Nosovskiĭ, Gleb Vladimirovich (1993). Geometrical and statistical methods of analysis of star configurations: dating Ptolemy's Almagest. CRC Press. p. 215. ISBN 0-8493-4483-2.
- Evans, James (1998). The History & Practice of Ancient Astronomy. Oxford University Press US. p. 259. ISBN 0-19-509539-1.
- Sarton, George (1993). Hellenistic science and culture in the last three centuries B.C. Courier Dover Publications. p. 53. ISBN 0-486-27740-2.
- Blue, Jennifer. "Gazetteer of Planetary Nomenclature". United States Geological Survey. Retrieved 2009-09-10.