‘‘The famous zenith sector’’ at Greenwich

Rebekah Higgitt1

School of History, University of Kent, United Kingdom

Curators are often asked which object from their collec-tions is their favourite but it is, of course, a very difficultquestion to answer. One can have any number of favour-ites for any number of reasons: things that are beautifulor complex, rare or unique, old or newly-acquired,oddities or things that represent important momentsfor individuals, groups or nations. The astronomy-relatedcollections at the National Maritime Museum (NMM) andRoyal Observatory, Greenwich (ROG), range from largetelescopes and quadrants to astrolabes, prints, orreriesand toys, and many of them have taken my fancy atdifferent times.

The object I have chosen on this occasion is not one ofthe beauties. Perhaps inspired by the British Museum’sHistory of the World in 100 Objects, I have chosen some-thing that can genuinely be said to have had an importantimpact on the history of practical astronomy, instrument-making in Britain and the Royal Observatory itself.However, one of the things that interests me most aboutthis particular object is its fluctuating fame. Today it is ondisplay to a large number of visitors but its history andpurpose is little known beyond specialist circles. Converse-ly, in the 18th and 19th centuries it was kept behind closeddoors and yet had something of a reputation with thepublic.

The object in question is the 12.5-foot zenith sectorthat the clock and instrument maker George Graham(c. 1673–1751), made in 1727 for James Bradley, thenSavilian Professor of Astronomy at Oxford. It is a long,wall-mounted telescope, with a short divided scale, usedto observe and measure the stars that appeared at thezenith, that is, directly overhead (Fig. 1). Observations atthe zenith are less affected by atmospheric aberrationthan those lower in the sky, allowing greater precisionin measuring their positions. As historian of science Jim

Lost and Found Endeavour Vol. 38 No. 3–4

Figure 1. The Bradley zenith sector (AST0992) at the Royal Observatory,

Greenwich, in the 1960s. It remains in the same position today.

B2614, � National Maritime Museum.

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Bennett has shown, this ‘‘celebrated’’ instrument sealedthe reputations of both men and, when Bradley becameAstronomer Royal and brought it to Greenwich in the1740s, it became part of a much-imitated suite of instru-ments that helped to establish the Royal Observatory’sreputation for accuracy.2

The zenith sector was famous because, unlike mostinstruments at the ROG, it can be associated with twoimportant discoveries. It was made in an attempt toobserve stellar parallax (from which the distance ofthe nearest stars could be calculated) but this proved to

1 Formerly at: Royal Museums Greenwich, United Kingdom.2 Bennett, J.A. (1992). ‘The English quadrant in Europe: instruments and the

growth of consensus in practical astronomy’, Journal of the History of Astronomy23(1), 1–14, p. 3. See also Howse, D. (1975). Greenwich Observatory. Vol. 3: Thebuildings and instruments. London: Taylor & Francis, pp. 60–64.

Available online 28 October 2014

www.sciencedirect.com 0160-9327/� 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/1

be impossible with even the very best 18th-centurytelescopes. However, Bradley’s observations at Kew andWanstead, near London, did lead to him confirming andexplaining an effect that he later called the ‘aberration oflight’, which is caused by the motion of the Earth combinedwith the finite velocity of light.3 Secondly, Bradleyobserved and accounted for the effects of the slight‘nodding’ of the Earth’s axis, called nutation. The formerwas the first observational proof that the Earth orbits theSun, but both discoveries were triumphs for the power andpossibilities of excellent instruments combined with goodhabits of observation. In addition, knowledge of these two

3 Fisher, J. (2010). ‘Conjectures and reputations: The composition and reception ofJames Bradley’s paper on the aberration of light with some reference to a thirdunpublished version’, British Journal for the History of Science 43(1), 19–48.

0.1016/j.endeavour.2014.09.007

Figure 2. The zenith sector as it appeared in the late 18th century, in a watercolour

drawing by John Charnock (PAF2940).

PW2940 detail, � National Maritime Museum.

Figure 3. The zenith sector at the Royal Observatory, Greenwich, in the 1890s,

mounted with other historic instruments on the wall of the Transit Circle Room.

� National Maritime Museum.

5 Brian Warner, B. (1979). Astronomers at the Royal Observatory Cape of Good

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effects led to a new degree of precision in astronomywherever it was practiced.

Bradley himself underlined the importance of the in-strument and its maker, writing in Philosophical Transac-tions in 1748 that any success he had in advancingastronomy ‘‘has principally been owing to the Adviceand Assistance given me by . . . Mr. George Graham’’.4

Shortly after, the perceived value of the zenith sectorwas confirmed when the Government bought it for £45and had it installed at Greenwich. It remained in regularuse there, correcting errors arising from any misalignmentof the mural quadrants, and undergoing various altera-tions, until a new instrument was brought into use in 1812(Fig. 2).

4 Bradley, J. (1748). ‘A Letter to the Right Honourable George Earl of Macclesfieldconcerning an Apparent Motion Observed in Some of the Fixed Stars’, PhilosophicalTransactions 45, 1–43, p. 6.

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Its useful life was not yet over, however, for in1837 the sector was sent to the Royal Observatory atthe Cape of Good Hope, where it was used by ThomasMaclear in re-surveying and extending the arc ofmeridian measured by Lacaille in 1750.5 The sectorwas returned to Greenwich in 1850 in good order, despitethe difficulties of using such a long and awkwardinstrument in the field. By that time it was, however,considered a relic rather than a scientific instrument.Like other old instruments that had once been key to theObservatory’s reputation, the zenith sector was hunglike a trophy on the wall of the new Transit Circle Room(Fig. 3).

In the 19th century, this particular instrument hadreached something approaching iconic status, even inpopular accounts of the Observatory. The Religious TractSociety’s newspaper, The Weekly Visitor, described it in1835 as ‘‘the famous zenith sector’’.6 Similarly, in1862 Edwin Dunkin, an assistant at the Observatory,informed readers of Leisure Hour magazine that ‘‘[e]veryperson who claims even the slightest acquaintance withastronomy, must possess some veneration’’ for theinstrument.7 Public admiration for the instrument wasparalleled by appreciation of Bradley’s abilities. Indeed,the Weekly Visitor article had named him ‘‘one of the

Hope. Cape Town and Rotterdam: A.A. Balkema, pp. 56–57.6 Anon. (1835, 3 February). The Weekly Visitor, p. 54.7 Edwin Dunkin, E. (1862, January). ‘The Royal Observatory, Greenwich: a day at

the observatory’, Leisure Hour, p. 23.

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greatest astronomers of his age’’ and, half a centurylater, the illustrated newspaper The Graphic stated thatbecause of ‘‘the revolution wrought in observational as-tronomy by his discoveries and researches’’ Newton wasright to have called Bradley the ‘‘best astronomer inEurope’’.8

Despite these impressive accolades in the past, how-ever, Bradley is largely forgotten today. The star objectsat the ROG are now, undoubtedly, John Harrison’smarine timekeepers, especially since the publication ofDava Sobel’s bestselling book on Harrison, Longitude(1995). Despite the presence of his influential Grahaminstruments, Bradley now receives comparatively littleattention. Since being highlighted on the Observatory’saudio guide a few years ago, the zenith sector, now

8 Anon. (1835, 3 February). The Weekly Visitor, p. 54; J.E.P., J.E. (1885, 8 August).‘The Royal Observatory, Greenwich’, The Graphic, p. 163.

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remounted near its working location in the MeridianObservatory, has benefitted from a little more attention,but it remains a difficult object for visitors to interpret; ahalf-forgotten Astronomer Royal made two confusing-sounding discoveries with a very long metal tube. Itis a leap to comprehend its impact on the potentialprecision of astronomical observation, let alone to graspthat the discovery of the aberration of light was thefirst observational proof of the heliocentric system.

I therefore urge visitors to the Observatory to standback (or at least, as far back as one can without bumpinginto Graham’s mural quadrant), to view ‘‘the famouszenith sector’’ and to recall that, even if it looks like arusty drainpipe, ‘‘there are few instruments, even now,having such a history’’.9

9 Lewis, T. (1890). ‘Notes on some historical instruments at the Royal Observatory,Greenwich’, The Observatory 163, 200–206, p. 202.