Kidinnu
Kidinnu (also Kidunnu) (circa 400 BC - circa 310 BC) wasChaldean astronomermathematician. StraboAmaseiaPontusPlinyElder called him Kidenas or Cidenas.Kidinnu was bornBabylon.
He wascontemporaryEudoxusCnidushis student CallippusCyzicus,principal ofastronomical school inBabylonian citySipparAkkad (now Abu Habbah, southwestBaghdad, Iraq).
Probably Kidinnu had made complex methodsequationscalculatingirregular movements ofMoonother planetsspecially ofSun. Because he was not as attached as Greek astronomers toconstant velocityplanets he was ableget good approximationstheir movements. ForSun,apparent angular velocity isminimumaphelion, whenEarthfarthest from it. So Kidinnu developed from Nabu-rimanni's System Amore refined system (now called System B) used byChaldean astronomersdescribe more clearlymotions ofSunplanets. This system used steadily increasingdecreasing values forplanetary positions, sometimes calledzigzag functions.
Forlength oftropical year Kidinnu used 365d 6h.
About 383 B.C Kidinnu obtained still more accurate valueslunar movements, first calculated before him by Nabu-rimanni. Formean length ofsynodic month he had already asyoung man mentionedvalueabout 29.530614d = 29d 12h 44 m 5san error less1s. The classical value29d 31:50:8:20 (sexagesimal) = 29d + 12h + 793/1080h = 29.53059414...dalso attributedhim;was confirmed by Hipparchusused by Ptolemylater astronomers.
Kidinnu probably introduced19-year cycle known asMetonic cycle intoBabylonian calendar383 BC. In this system each year had 12 lunar months,7 extra months were inserted at intervals during19-year period. This cycle, withvalue formean synodic month, was later adopted forHebrew calendarhas remaineduse until today.
Chaldean astronomers had also found changes ofapparent lunar diameter. They had ascertained thatlunar diameter changes from 29' 30"34' 16". Actual values29' 30"32' 55", whichvery close. Itnot known yet ifknew aboutchangesapparent solar diameter, which was later known by SosigenesAlexandria.
In about 314 BC Kidinnu knew thatsidereal year was longer thantropical yeartherefore may have been aware ofprecession ofequinoxes. He had smoothedway formore accurate calculationsHipparchus sinceseems that Hipparchus, who may have worked forwhileBabylon (near Kidinnu's citySippar) around 139 BC, knew Kidinnu's work. Kidinnu also influenced Claudius Ptolemy.
