The Greenwich Meridian

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Halley’s 5-foot Transit Instrument

This spot marks the original site of Halley’s 5-foot Transit Instrument – the instrument that defined the Greenwich Meridian from 1721 until replaced by Bradley’s 8-foot Transit Instrument in 1750. In 1721, the Observatory occupied a much smaller site than it does today. Halley set the telescope up in a small gap between the western boundary wall and Flamsteed House. The instrument was moved to what is now called the Meridian Building as a relic in 1818, and the original site subsumed into Flamsteed House. There are no known images of the telescope when in its original location. Nor are there any images in which the boarded shed built to cover it can be seen in contemporary illustrations (see for example, the image shown here from 1749). In today’s Observatory, the site where the telescope would originally have been located, is just behind the door to the right at the bottom of the stairs leading down from the Octagon Room in Flamsteed House. The telescope itself is on display in the Meridian Building.

The maker

The maker of the telescope is uncertain. Writing in 1764, Lalande stated that it was made by Robert Hooke. Although Hooke himself had died in 1702, it is not impossible that an existing Hooke telescope was fitted with the appropriate axis and braces by George Graham.

The recorded observations

The first recorded observation was made on 1 October 1721. Regular observations ceased in 1725 following the commissioning of the 8-foot Iron Mural Quadrant. The last recorded observation by Halley was made in 1730. His successor Bradley made the first in a regular series of over 25,000 observations in January 1743 and the last on 2 September 1750 when the telescope was replaced.

A troublesome design

Unlike the later transit telescopes, the Halley Instrument was mounted off centre on its supporting axis. The telescope to pivot distance is 34¼ inches on one side and 7¾ inches on the other. The western pivot was supported on a mount set into the boundary wall and the eastern one on a freestanding pier. The reason for the offset from the centre of the axis is unclear. The gap in which the telescope was located was certainly awkward, being orientated at an angle of about 13½º to the Meridian. Hornsby, who edited Bradley’s observation before they were published, thought the offset arose as a result of the awkward position, as did Maskelyne. Later commentators have argued that it was designed like this to prevent flexure of the axis. What is certainly true is that other early transit instruments, including that made by their inventor Römer in 1684 were offset in a similar way. Whatever the reason for the offset with the Greenwich instrument, it consequence was to make it difficult to check the alignment of the optics (collimation errors) by reversing the telescope in its mountings – something that became standard practice with the two successor instruments.

The difficulty of collimation

In his History of Physical Astronomy (1852), Robert Grant described the adjustments Römer made to his transit instrument:

The error of collimation of the telescope was ascertained by reversing the instrument. For this purpose a [double] mark was carefully observed in the open fields before the instrument was placed in its position. … . As this mode of verification was troublesome he subsequently effected the same object by observing two distant marks in the horizon, diametrically opposite to each other.

In Halley’s time, the Greenwich instrument only appears only to have been used with a bespoke southern mark. This was located on the wall of Greenwich Park. Bradley took up residence as Astronomer Royal in June 1742 and in about October 1743, set up a bespoke northern mark (or marks?) on a chimney in Greenwich to supplement it.

On the 3 August 1742, Bradley described how he collimated the telescope with the use of a double mark in the manner of Römer. Although Bradley would undoubtedly have preferred to create the double mark where he already had an existing mark on the park wall to the south, the restricted site in which the telescope was located would have prevented the tube from being lowered to the horizon when in the reversed position. Instead, he had to set up a temporary pair of marks to the north.

… The marks by which I adjusted the line of collimation were two pieces of paper each two inches broad pasted on a board 26½ inches apart (the middle of the telescope being 26½ inches nearer to one end of the axis than the other) The board with the marks on it was fixed up beyond the pouder house at a distance of a little more than a mile from ye observatory: as I collected from the Angle subtended by the two marks which (with ye micrometer in ye 15f tube) I found to be 1’.23”, which gives a distance of the board 5488 feet or 208 feet above a mile. …

The ‘pouder house’ is presumed to be the building on the banks of the Thames labelled ‘The New Magazine’ by Travers in his map A survey of the Kings Lordship or Manor of East Greenwich in the county of Kent (1695) and ‘Magazine’ by Roque in his Survey of London (1741–45). Both maps indicate it stood 0.8–0.9 miles north of the Observatory (roughly where Enderby Wharf is to be found today). On the river as contained by its banks today, the meridian of the telescope would have cut an arc across the curving Thames, entering the river at a distance of about 2,770 feet and leaving it at a distance of around 7,890 feet (at the Victoria deep water wharf). This suggests either: that Bradley underestimated the distance by as much as 30% (the exact amount being dependent on how the position of the river banks have changed since Bradley’s time), or that the marks were set up at a point that was significantly to the east of the meridian. That Bradley might have underestimated the distance is not as implausible, as seven years later he was to underestimate the distance to the south mark for his new transit telescope by around 12%. If the twin marks really were set up at a distance of 5,488 feet, then in order to view them, the telescope would have had to be turned though an angle of about 10º to the east – something that would have required some sort of temporary mounting either within the Observatory or perhaps outside. As well as not recording the location or orientation of the telescope during the collimation process, Bradley also did not explain how the double mark was adjusted to be perpendicular to the line of the telescope, as the spacing between the two marks would have required. Bradley makes few references to the double marks, but a reference on 6 June 1744 to them being ‘placed as usual beyond the pouder house’ suggests that they were set up on a regular basis. On 25 September 1744. he appears to have been having a bad hair day as the marks had to be set up twice on the same day.

Although the cumbersome procedure with the double mark could not be entirely abandoned following the erection of the north mark in October 1743, checking the collimation undoubtedly became much easier.