There are two ways to measure the Moon’s “month”. This can be confusing, even for many adults.

The sidereal orbital period of approximately 27.3 days is the time the Moon needs to complete one orbit around the Earth, measured relative to the stars. Then there is the synodic month of approximately 29.5 days: the time between two full moons. That difference of just over two days arises because the Earth itself has also travelled some distance around the Sun in the meantime, meaning the Moon has to travel a little further in order to show the same phase again to someone looking up from Earth.

And there is something else remarkable about the Moon’s orbit: we always see the same side. This is because the Moon rotates exactly once on its own axis in the time it takes to complete one orbit around the Earth — a phenomenon known as synchronous rotation (or tidal locking). As a result, the far side of the Moon remained invisible and unknown for centuries, until spaceflight made it possible to capture images of that side for the first time.

The synodic month of around 29.5 days forms the basis of many traditional lunar calendars around the world. The fact that the word “month” and the word “moon” are closely related in many languages is no coincidence — though there is no need to overthink it: our ancestors simply looked up, counted the days between two full moons, and had themselves a calendar.