The metonic cycle refers to a period of 19 years, which is the (approximate) least common multiple of days between a solar year (about 365.25 days) and a lunar month (about 29.5 days). Surprisingly, 19 solar years differs from 235 lunar months by only about 2 hours!

So, why is this relevant? Its Wikipedia entry yields a very interesting juxtaposition: “The cycle’s most significant contemporary use is to help in flight planning (trajectory calculations and launch window analysis) for lunar spacecraft missions as well as serving as the basis for the Hebrew calendar’s 19 year cycle.”

Nothing like bringing together the ancient and the modern through calendrical calculations.