The Greenwich Meridian

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Ground based marks accurately placed on the meridian were regularly used by the Greenwich astronomers as a quick means of determining the alignment errors of their transit instruments. Ideally, both a northern and a southern mark were used.

The first recorded observation with the Halley 5-foot Transit Instrument was made on 1 October 1721. In Halley’s time, the instrument appears only to have been used with a bespoke southern mark, located on the wall of Greenwich Park. In about October 1743, Bradley set up a northern mark (location unknown) on a chimney in Greenwich. However, recording the events of 1 September 1726, Bradley wrote:

I went to Greenwich; and Mr. Graham and I adjusted the meridian telescope at right angles to the axis. […] After the line of collimation was truly at right angles to the axis, we set [the] thread exactly to the mark made for a meridian on the park wall, near Admiral Hosier’s house, and then looking backwards towards the north, we found that the left side of the wire just touched the extreme point of the top of a house belonging to some hospital in Greenwich, leaving the said point to the left hand of the thread. It likewise cut a light in a garret window of some house in the town, so as to make the part that was to the right of the thread a small matter less than the other. (SP Rigaud: Some particulars respecting the principal Instruments at the Royal Observatory at Greenwich, in the time of Dr. Halley, Memoirs of Royal Astronomical Society Vol lX (1836) p205-227).

The hospital referred to is assumed to be Trinity Hospital. Built in 1613, it was originally known as the Norfolk Hospital or Northampton Almshouses. It was remodelled in the 'gothic' style in 1812.

The 1796 view by JP Malcolm comes from Environs of London (1796) by Daniel Lysons and shows the building much as it would have appeared in Bradley’s time, though from the opposite side to the Observatory. The ‘extreme point’ to which Bradley refers has been marked. The c.1675 and c.1905 views show the hospital from the Observatory side. In the earlier view, it is the second building from the right fronting the river. In the later view, which is similar but not the same as it would have been from where the telescope was located, the position of the ‘extreme point’ has again been marked. The view of the hospital is now blocked from the Observatory by later buildings.

Writing on 10 October 1744, Bradley recorded: ‘About this time the middle limb of the garret window in Greenwich with which ye telescope was sometimes compared was taken down and a new one put in more easterly’. Like the chimney on which Bradley set up his permanent north mark, the location of the garret window remains unknown.