The Royal Greenwich Observatory built the Astronomical Clock inside the York Minster in 1955 to commemorate the 18,000 RAF aircrew who died in World War II. As well as timekeeping, it incorporates information vital to pilots flying over occupied Europe. The Zodiacal face of the clock shows the positions of circumpolar stars, those which never set below the horizon. Sidereal or star time is shown at the top of the dial and by three small dials at the left, where it has been converted from hours to degrees.