A key instrument for determining time was the Airy Transit Circle, which was used primarily from 1851 to 1938. It was agreed in 1884 that the "meridian line marked by the cross-hairs in the Airy Transit Circle eyepiece would indicate 0° longitude and the start of the Universal Day" according to RMG. The time is determined by marking the time a star of known location would pass through the aimpoint of the telescope. In a reverse case, this type of instrument was also used for making star charts.