|Royal Observatory, Meridian Building (on Bradley Meridian)|
|Distance (S) from Greenwich|
|OS map details||OS Explorer: 161 or 162|
|OS grid ref||TQ 38877.77324|
|WGS84 lat/long||51.477835, -0.001588|
|Marking date||1750 & 1816|
|Access||Restricted (charged entry)|
Between 1750 and 1850, the Greenwich Meridian was defined by one or other of the two telescopes that between them occupied this spot. The instrument occupying the spot today is the later Troughton 10-foot Transit Instrument, which defined the Meridian from 1816 until 1850. It replaced the earlier Bradley 8-foot Transit Instrument of 1750, which today, is on display in the glass case immediately to the west. To accommodate the extra length of the Troughton instrument (which also had a shorter axis), semicircular caps, 2 feet in diameter and 2 feet 3 inches long were placed on top of the existing piers. The position of the Greenwich Meridian moved in 1851, when the Troughton instrument was superseded by the Airy Transit Circle (ATC), roughly 6 metres to its west. The Meridian defined by the Bradley and Troughton instruments has been known as the Bradley Meridian since at least 1888, when the then Astronomer Royal, William Christie, referred to it as ‘the old meridian of Bradley’.