Sunday, October 11, 2015

Moon: Timocharis

Timocharis crater
"AS15-M-1147". Licensed under Public Domain via Commons

Timocharis Crater
Timocharis is a prominent lunar impact crater located on the Mare Imbrium. The closest crater of comparable dimensions is Lambert to the west. The smaller craters Feuillée and Beer lie to the east of Timocharis.

The rim of Timocharis has a somewhat polygonal outline, with an outer rampart that extends over 20 kilometers in all directions. The interior wall is slumped and sharply terraced. The center of the floor is occupied by a craterlet that lies on a slight rise. This impact has almost completely removed the original central peak.

The crater may have a minor ray system that extends for over 130 km (81 mi). The lack of prominent rays puts the age of this crater at about a billion years or more.

To the north of Timocharis is a tiny crater chain named the Catena Timocharis.

Timocharis of Alexandria
Timocharis of Alexandria (Greek: Τιμόχαρις; c. 320 – 260 BC) was a Greek astronomer and philosopher. Likely born in Alexandria, he was a contemporary of Euclid.

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 BC.

Between 295 and 272 BC,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 BC when the planet came within 15 arcminutes of the star η Virginis.

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.

Venus and Spica
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.

Precession of the equinoxes
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.

Excerpts from Wiki texts have been incorporated into the blog as kinds of abstracts for reader's convenience. By clicking the links much more can be learned about these subjects.

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