A unit of time equal to 19 years, used in astronomy in predicting the phases of the Moon. By coincidence, 19 years is equal to 6939.602 days, and 235 lunar months is equal to 6939.689 days--just 125 minutes longer. As a result, the phases of the Moon repeat almost exactly after 19 years. (Nineteen years can contain either 3 or 4 leap days, so the recurrence isn't always exact to the day of the month.) Named for the ancient Greek astronomer Meton, who first used it for predictions around 433 BC.