Why Do the Months Have Different Numbers of Days?

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Months have different numbers of days because a month is a measure of time that corresponds or nearly corresponds to the length of time the Moon revolves once around Earth. The lengths of all astronomical months vary slightly due to perturbations in the Moon’s orbit.

The synodic month, which is the complete cycle of phases of the Moon as seen from Earth, is on average 29 days 12 hours 44 minutes 3 seconds. According to Encyclopedia Britannica, a lunar-based year and calendars derived from it, such as the currently used Gregorian calendar cannot be accurately correlated with a solar-based year, so the continued use of the Gregorian calendar is merely recognition of its convenience as a calendar division