Kidinnu (also Kidunnu; possibly fl. 4th century BC; possibly died 14 August 330 BC) was a Chaldean astronomer and mathematician. Strabo of Amaseia called him Kidenas, Pliny the Elder called him Cidenas, and Vettius Valens called him Kidynas. Some cuneiform and classical Greek and Latin texts mention an astronomer with this name, but it is not clear if they all refer to the same individual: The Greek geographer Strabo of Amaseia, in Geography 16.1.6, writes: "In Babylon a settlement is set apart for the local philosophers, the Chaldeans, as they are called, who are concerned mostly with astronomy; but some of these, who are not approved of by the others, profess to be writers of horoscopes. (There is also a tribe of the Chaldeans, and a territory inhabited by them, in the neighborhood of the Arabs and of the Persian Gulf, as it is called.) There are also several tribes of the Chaldean astronomers. For example, some are called Orcheni [those from Uruk], others Borsippeni [those from Borsippa], and several others by different names, as though divided into different sects which hold to various dogmas about the same subjects. And the mathematicians make mention of some of these men; as, for example, Kidenas, Nabourianos and Soudines". The Roman encyclopedist Pliny the Elder, in Natural History II.vi.39, writes that the planet Mercury can be viewed "sometimes before sunrise and sometimes after sunset, but according to Cidenas and Sosigenes never more than 22 degrees away from the sun". The Roman astrologer Vettius Valens, in Anthology, says that he used Hipparchus for the Sun, Sudines and Kidynas and Apollonius for the Moon, and again Apollonius for both types (of eclipses, i.e. solar and lunar). The Hellenistic astronomer Ptolemy, in Almagest IV 2, discusses the duration and ratios of several periods related to the Moon, as known to "ancient astronomers" and "the Chaldeans" and improved by Hipparchus. He mentions (at H272) the equality of 251 (synodic) months to 269 returns in anomaly. In a preserved classical manuscript of the excerpt known as Handy Tables, an anonymous reader in the third century wrote the comment (a scholium) that Kidenas discovered this relation. The colophon of two Babylonian System B lunar ephemerides from Babylon (see ACT 122 for 104–101 BC, and ACT 123a for an unknown year) say that they are the tersitu of Kidinnu. A damaged cuneiform astronomical diary tablet from Babylon (Babylonian Chronicle 8: