The celebrated Queens’ College Moon Dial, often erroneously attributed to Sir Isaac Newton, was painted on an exterior wall at Cambridge University in 1733. In addition to the traditional solar hours, the timepiece has an accompanying table that can be used to read the shadow cast by the light of the moon.

Even with the table, however, a series of four mathematical calculations has to be carried out by anyone wishing to know the time. “The moon’s motion is so irregular that no moon-dial can possibly be accurate, and is to be regarded as providing an exercise in mental arithmetic rather than being an instrument of any practical value,” writes the author of a short booklet on the dial.