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 following information is an excerpt of the overview of a century of scholarship in the sources referenced below.
The meaning of tersitu is not known definitively. Already Franz Xaver Kugler proposed that the word can be interpreted here as "table"; in another context it seems to mean something like "tool", but in yet another it refers to a blue enamel paste. P. Schnabel, in a series of papers (1923–1927), interpreted the phrase as an assignment of authorship. He argued that Naburimannu developed the Babylonian System A of calculating Solar System ephemerides, and that later Kidinnu developed the Babylonian System B. A Greco-Roman tradition, mentioned above, attributes to Kidinnu the discovery that 251 synodic months equals 269 anomalistic months. This relationship is implicit in System B, and is therefore another reason to believe that Kidinnu was involved in developing the lunar theory of System B. However, the conclusion that Kidinnu is the main creator of System B is uncertain. Babylonian astronomers before Kidinnu's time apparently already knew the Saros cycle (old eclipse observations were collected in tables organised according to the Saros cycle since the late 5th century BC) and the Metonic cycle (the dates of the lunar calendar in the Saros tables follow a regular 19-year pattern of embolismic months at least since 498 BC); both cycles are also used in System B. Schnabel computed specific years (first 314 BC and later 379 BC) for the origin of the System B lunar theory, but Franz Xaver Kugler and Otto E. Neugebauer later disproved Schnabel's calculations. Schnabel also asserted that Kidinnu discovered precession when distinguishing between sidereal and tropical years; Neugebauer contested this and current scholarship considers this conclusion to be unfounded.
The lunation length used in System B has also been attributed to Kidinnu. It is 29 days + 191 time degrees + 1/72 of a time degree ("barley corn") = 29d 31:50:8:20 (sexagesimal) = 29d + 12h + 793/1080h (Hebrew helek) = 29.53059414...d. Being a rounded value in the archaic unit of "barley corns" it may be even more ancient. In any case, it is very accurate, within about ⅓ of a second per month. Hipparchus confirmed this value for the lunation length. Ptolemy accepted and used it, as mentioned above. Hillel II first used it in the Hebrew calendar, and it has been used for that purpose ever since.
The existing evidence makes it difficult to put Kidinnu at a time and place. Schnabel placed Kidinnu in Sippar, but Otto E. Neugebauer showed that Schnabel based this conclusion on a misreading of the cuneiform tablet. Classical sources like Strabo mention different "schools" and "doctrines" followed in different places (Babylon, Borsippa, Sippar, Uruk). System A and B have been used contemporaneously, and tablets for both systems have been found in both Babylon and Uruk. Tablets based on System B, associated with Kidinnu, have been found mostly in Uruk, but the earlier tablets came predominantly from Babylon. The oldest preserved tablet using System B comes from Babylon and dates from 258 to 257 BC. This is in the Seleucid era, but it is plausible that the traditional Chaldean astronomical systems had been developed before the Hellenistic period. The Alexander chronicle mentioned above suggests that the famous astronomer Kidinnu died in Babylon in 330 BC, if it refers to the same Kidinnu who was mentioned on the ephemeris tablets centuries later.
A History of Ancient Mathematical Astronomy Part One II Intr. 3.1 (pp. 354–357), Part Two IV A 4, 3A (p. 602) and IV A 4, 4A (pp. 610–612). Springer, Heidelberg 1975 (reprinted 2004).