Ask Tom Why

Dear Tom,
My son was born on a leap day ---Feb. 29, 2000 --- and he is a budding astronomer. He took great interest in the Dec. 31, 2009, blue moon, and he knows a blue moon can occur only at the end of a month. He wonders when there will be a blue moon in February, hopefully on his birthday.
— Sandra Schulz
Dear Sandra,
A blue moon is popularly defined as the second full moon in a given month. A full moon rolls around every 29.5 days (actually it’s 29.5306 days).
A consequence of that interval is that a full moon on the first or second day of a calendar month will be followed by a second full moon — the blue moon — 29.5 days later in the same month.
Another consequence is that February, with 28 days (or 29 in a leap year) can never have a blue moon. We hope this information won’t disappoint your son.