The Moon's cycle of roughly 29 days gave rise to the length of a month.

The longer version

The Moon returns to the same phase roughly every 29.5 days, because after completing its orbit Earth has also moved along its own, so the Moon must travel a little further to catch the same angle of sunlight. That span became the month. Waxing and waning is easy to observe without instruments.

Calendars based directly on lunar cycles run about eleven days short of the solar year, drifting through the seasons unless an extra month is inserted periodically. Modern civil months no longer track the phases at all; they are administrative slices of a solar year that merely keep the old name. Many religious calendars still follow the phases closely.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.