Sidereal months

A sidereal month is the length of time it takes for the Moon to return to the same position amongst the stars, hence its name, which comes from the Latin word for stars, sidera.

A sidereal month lasts between 27 and 28 days. They are often shown in tables by the letters a–z repeated, as on the Zeno Wheel. To account for the extra letters needed to make the month the correct length, extra letters are often added after the initial z, as seen before the blue As in the above image.