METONIC CYCLE, in chronology, a period of 19 years during which there are 235 lunations, so called because discovered by Meton. Computation from modern data shows that 235 lunations are 6939 days, 16·5 hours; and 19 solar years, 6939 days, 14·5 hours. The relation between integral numbers of months and years expressed by Meton’s rule therefore deviates only two hours from the truth. Since 19 Julian years make 6939 days, 18 hours, the relation errs by only 1·5 hour when the Julian year is taken. Meton was an Athenian astronomer (fl. 432 B.C.).