Perpetual Moon phases calendar with a Tidal abacus or computer.This Elisabethean instrument was conceived to address the need to predict the time of tidal changes. It draws on their intimate association with the rotation of the Moon, which influences the tides as it revolves around the Earth.Knowledge of the lunar phases has been of great importance for many cultures: calendars, religious festivals and agricultural practices, were set according to the moon. The Tidal Abacus, or calculator, is employed to predict the time of tidal changes.