Historical records from around the world show continued efforts to build reliable lunisolar calendars. One of the methods used most often was to insert (intercalate) an extra month every few years. It might be done randomly, but usually it is not. In the fifth century (BCE) a Greek astronomer Meton set down specific rules for inserting these extra months. If one picks a year that starts with a New Moon and lets the months run in sequence, how many lunar months pass before another year comes that starts on a New Moon? The answer is 235 lunar months - or 19 years. In other words the Full Moon appears on the same day in that year as it did 19 years earlier. This 19 year period defines the
Metonic Cycle. This cycle is useful for calendar makers. The same pattern of lunar phase and date in the year repeats every 19 years. A calendar maker needs to follow only one pattern - change the number of months for a prechosen pattern of years and repeat that pattern every 19 years. This number was so important to ancient calendar makers that the Greeks inscribed this number in golden letters on a temple in Athens - hence the term
The Golden Number, G. Today's almanacs, including
The Astronomical Almanac, provide The Golden Number. As it turns out, however, the Metonic Cycle is not quite exactly 19 years. It is off by about 2 hours per cycle.