Reverse of a bras perpetual calendar made by Hartston which was used for determining the dates of Easter in the Julian and Gregorian calendars. The Julian calendar was established in 46 BC by Julius Caesar on the premise that the solar year was 365.25 days long, whereas it is in fact 365.24219 days long: an inaccuracy which after hundreds of years caused astronomical phenomena to shift backwards against the calendar. The Gregorian calendar was instituted in 1582 by Pope Gregory XIII to reform the Julian calendar, through the introduction of the leap year system, which is still used in most of the world today. Protestant nations such as England were slow to accept the new calendar, which probably accounts for the dual nature of this calendar.
© Science Museum / Science & Society Picture Library