February's Last-Quarter Moon: Why It's Missing

The mathematics behind this is that the average synodic lunar month — from new moon to new moon — is 29.53 days long, while February is either 28 or 29 days long. We make a big fuss about the "Blue Moon," when there are two full moons in a month, but we don’t seem to notice when one of the lunar phases goes missing. The lunar month consists of 29.530589 days, and the tropical year (equinox to equinox) is 365.242190 days long.