Printed paper volvelle, one of two, for Mercury, framed, published by Leonhardt Thurneisser, Germany, 1574-1575

A volvelle is a series of movable discs used to calculate the position and phases of the planets or the rising and setting of the Sun. This volvelle is used to calculate the position of Mercury in the period 1575-1580. Astrology was important for some medical traditions such as Paracelsian medicine. It was believed that the planets could influence the body and the humours. It was also believed that the positioning of the planets could determine the most opportune time for treatment.