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“The famous zenith sector” at Greenwich
Lost and Found
Endeavour
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Vol. 38 No. 3–4
ScienceDirect
‘‘The famous zenith sector’’ at Greenwich Rebekah Higgitt1 School of History, University of Kent, United Kingdom
Curators are often asked which object from their collections is their favourite but it is, of course, a very difficult question to answer. One can have any number of favourites for any number of reasons: things that are beautiful or complex, rare or unique, old or newly-acquired, oddities or things that represent important moments for individuals, groups or nations. The astronomy-related collections at the National Maritime Museum (NMM) and Royal Observatory, Greenwich (ROG), range from large telescopes and quadrants to astrolabes, prints, orreries and toys, and many of them have taken my fancy at different times. The object I have chosen on this occasion is not one of the beauties. Perhaps inspired by the British Museum’s History of the World in 100 Objects, I have chosen something that can genuinely be said to have had an important impact on the history of practical astronomy, instrumentmaking in Britain and the Royal Observatory itself. However, one of the things that interests me most about this particular object is its fluctuating fame. Today it is on display to a large number of visitors but its history and purpose is little known beyond specialist circles. Conversely, in the 18th and 19th centuries it was kept behind closed doors and yet had something of a reputation with the public. The object in question is the 12.5-foot zenith sector that the clock and instrument maker George Graham (c. 1673–1751), made in 1727 for James Bradley, then Savilian Professor of Astronomy at Oxford. It is a long, wall-mounted telescope, with a short divided scale, used to observe and measure the stars that appeared at the zenith, that is, directly overhead (Fig. 1). Observations at the zenith are less affected by atmospheric aberration than those lower in the sky, allowing greater precision in measuring their positions. As historian of science Jim Bennett has shown, this ‘‘celebrated’’ instrument sealed the reputations of both men and, when Bradley became Astronomer Royal and brought it to Greenwich in the 1740s, it became part of a much-imitated suite of instruments that helped to establish the Royal Observatory’s reputation for accuracy.2 The zenith sector was famous because, unlike most instruments at the ROG, it can be associated with two important discoveries. It was made in an attempt to observe stellar parallax (from which the distance of the nearest stars could be calculated) but this proved to 1
Formerly at: Royal Museums Greenwich, United Kingdom. Bennett, J.A. (1992). ‘The English quadrant in Europe: instruments and the growth of consensus in practical astronomy’, Journal of the History of Astronomy 23(1), 1–14, p. 3. See also Howse, D. (1975). Greenwich Observatory. Vol. 3: The buildings and instruments. London: Taylor & Francis, pp. 60–64. Available online 28 October 2014
Figure 1. The Bradley zenith sector (AST0992) at the Royal Observatory, Greenwich, in the 1960s. It remains in the same position today. B2614, ß National Maritime Museum.
be impossible with even the very best 18th-century telescopes. However, Bradley’s observations at Kew and Wanstead, near London, did lead to him confirming and explaining an effect that he later called the ‘aberration of light’, which is caused by the motion of the Earth combined with the finite velocity of light.3 Secondly, Bradley observed and accounted for the effects of the slight ‘nodding’ of the Earth’s axis, called nutation. The former was the first observational proof that the Earth orbits the Sun, but both discoveries were triumphs for the power and possibilities of excellent instruments combined with good habits of observation. In addition, knowledge of these two
2
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3 Fisher, J. (2010). ‘Conjectures and reputations: The composition and reception of James Bradley’s paper on the aberration of light with some reference to a third unpublished version’, British Journal for the History of Science 43(1), 19–48.
0160-9327/ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.endeavour.2014.09.007
Lost and Found
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Vol. 38 No. 3–4
155
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. ß National Maritime Museum.
Figure 2. The zenith sector as it appeared in the late 18th century, in a watercolour drawing by John Charnock (PAF2940). PW2940 detail, ß National Maritime Museum.
effects led to a new degree of precision in astronomy wherever it was practiced. Bradley himself underlined the importance of the instrument and its maker, writing in Philosophical Transactions in 1748 that any success he had in advancing astronomy ‘‘has principally been owing to the Advice and Assistance given me by . . . Mr. George Graham’’.4 Shortly after, the perceived value of the zenith sector was confirmed when the Government bought it for £45 and had it installed at Greenwich. It remained in regular use there, correcting errors arising from any misalignment of the mural quadrants, and undergoing various alterations, 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 Macclesfield concerning an Apparent Motion Observed in Some of the Fixed Stars’, Philosophical Transactions 45, 1–43, p. 6.
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Its useful life was not yet over, however, for in 1837 the sector was sent to the Royal Observatory at the Cape of Good Hope, where it was used by Thomas Maclear in re-surveying and extending the arc of meridian measured by Lacaille in 1750.5 The sector was returned to Greenwich in 1850 in good order, despite the difficulties of using such a long and awkward instrument 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 the Observatory’s reputation, the zenith sector was hung like a trophy on the wall of the new Transit Circle Room (Fig. 3). In the 19th century, this particular instrument had reached something approaching iconic status, even in popular accounts of the Observatory. The Religious Tract Society’s newspaper, The Weekly Visitor, described it in 1835 as ‘‘the famous zenith sector’’.6 Similarly, in 1862 Edwin Dunkin, an assistant at the Observatory, informed readers of Leisure Hour magazine that ‘‘[e]very person who claims even the slightest acquaintance with astronomy, must possess some veneration’’ for the instrument.7 Public admiration for the instrument was paralleled by appreciation of Bradley’s abilities. Indeed, the Weekly Visitor article had named him ‘‘one of the 5 Brian Warner, B. (1979). Astronomers at the Royal Observatory Cape of Good 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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Endeavour Vol. 38 No. 3–4
greatest astronomers of his age’’ and, half a century later, the illustrated newspaper The Graphic stated that because of ‘‘the revolution wrought in observational astronomy by his discoveries and researches’’ Newton was right to have called Bradley the ‘‘best astronomer in Europe’’.8 Despite these impressive accolades in the past, however, Bradley is largely forgotten today. The star objects at the ROG are now, undoubtedly, John Harrison’s marine timekeepers, especially since the publication of Dava Sobel’s bestselling book on Harrison, Longitude (1995). Despite the presence of his influential Graham instruments, Bradley now receives comparatively little attention. Since being highlighted on the Observatory’s audio guide a few years ago, the zenith sector, now
remounted near its working location in the Meridian Observatory, has benefitted from a little more attention, but it remains a difficult object for visitors to interpret; a half-forgotten Astronomer Royal made two confusingsounding discoveries with a very long metal tube. It is a leap to comprehend its impact on the potential precision of astronomical observation, let alone to grasp that the discovery of the aberration of light was the first observational proof of the heliocentric system. I therefore urge visitors to the Observatory to stand back (or at least, as far back as one can without bumping into Graham’s mural quadrant), to view ‘‘the famous zenith sector’’ and to recall that, even if it looks like a rusty drainpipe, ‘‘there are few instruments, even now, having such a history’’.9
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.
9 Lewis, T. (1890). ‘Notes on some historical instruments at the Royal Observatory, Greenwich’, The Observatory 163, 200–206, p. 202.
www.sciencedirect.com
Endeavour
Full text provided by www.sciencedirect.com
Vol. 38 No. 3–4
ScienceDirect
‘‘The famous zenith sector’’ at Greenwich Rebekah Higgitt1 School of History, University of Kent, United Kingdom
Curators are often asked which object from their collections is their favourite but it is, of course, a very difficult question to answer. One can have any number of favourites for any number of reasons: things that are beautiful or complex, rare or unique, old or newly-acquired, oddities or things that represent important moments for individuals, groups or nations. The astronomy-related collections at the National Maritime Museum (NMM) and Royal Observatory, Greenwich (ROG), range from large telescopes and quadrants to astrolabes, prints, orreries and toys, and many of them have taken my fancy at different times. The object I have chosen on this occasion is not one of the beauties. Perhaps inspired by the British Museum’s History of the World in 100 Objects, I have chosen something that can genuinely be said to have had an important impact on the history of practical astronomy, instrumentmaking in Britain and the Royal Observatory itself. However, one of the things that interests me most about this particular object is its fluctuating fame. Today it is on display to a large number of visitors but its history and purpose is little known beyond specialist circles. Conversely, in the 18th and 19th centuries it was kept behind closed doors and yet had something of a reputation with the public. The object in question is the 12.5-foot zenith sector that the clock and instrument maker George Graham (c. 1673–1751), made in 1727 for James Bradley, then Savilian Professor of Astronomy at Oxford. It is a long, wall-mounted telescope, with a short divided scale, used to observe and measure the stars that appeared at the zenith, that is, directly overhead (Fig. 1). Observations at the zenith are less affected by atmospheric aberration than those lower in the sky, allowing greater precision in measuring their positions. As historian of science Jim Bennett has shown, this ‘‘celebrated’’ instrument sealed the reputations of both men and, when Bradley became Astronomer Royal and brought it to Greenwich in the 1740s, it became part of a much-imitated suite of instruments that helped to establish the Royal Observatory’s reputation for accuracy.2 The zenith sector was famous because, unlike most instruments at the ROG, it can be associated with two important discoveries. It was made in an attempt to observe stellar parallax (from which the distance of the nearest stars could be calculated) but this proved to 1
Formerly at: Royal Museums Greenwich, United Kingdom. Bennett, J.A. (1992). ‘The English quadrant in Europe: instruments and the growth of consensus in practical astronomy’, Journal of the History of Astronomy 23(1), 1–14, p. 3. See also Howse, D. (1975). Greenwich Observatory. Vol. 3: The buildings and instruments. London: Taylor & Francis, pp. 60–64. Available online 28 October 2014
Figure 1. The Bradley zenith sector (AST0992) at the Royal Observatory, Greenwich, in the 1960s. It remains in the same position today. B2614, ß National Maritime Museum.
be impossible with even the very best 18th-century telescopes. However, Bradley’s observations at Kew and Wanstead, near London, did lead to him confirming and explaining an effect that he later called the ‘aberration of light’, which is caused by the motion of the Earth combined with the finite velocity of light.3 Secondly, Bradley observed and accounted for the effects of the slight ‘nodding’ of the Earth’s axis, called nutation. The former was the first observational proof that the Earth orbits the Sun, but both discoveries were triumphs for the power and possibilities of excellent instruments combined with good habits of observation. In addition, knowledge of these two
2
www.sciencedirect.com
3 Fisher, J. (2010). ‘Conjectures and reputations: The composition and reception of James Bradley’s paper on the aberration of light with some reference to a third unpublished version’, British Journal for the History of Science 43(1), 19–48.
0160-9327/ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.endeavour.2014.09.007
Lost and Found
Endeavour
Vol. 38 No. 3–4
155
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. ß National Maritime Museum.
Figure 2. The zenith sector as it appeared in the late 18th century, in a watercolour drawing by John Charnock (PAF2940). PW2940 detail, ß National Maritime Museum.
effects led to a new degree of precision in astronomy wherever it was practiced. Bradley himself underlined the importance of the instrument and its maker, writing in Philosophical Transactions in 1748 that any success he had in advancing astronomy ‘‘has principally been owing to the Advice and Assistance given me by . . . Mr. George Graham’’.4 Shortly after, the perceived value of the zenith sector was confirmed when the Government bought it for £45 and had it installed at Greenwich. It remained in regular use there, correcting errors arising from any misalignment of the mural quadrants, and undergoing various alterations, 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 Macclesfield concerning an Apparent Motion Observed in Some of the Fixed Stars’, Philosophical Transactions 45, 1–43, p. 6.
www.sciencedirect.com
Its useful life was not yet over, however, for in 1837 the sector was sent to the Royal Observatory at the Cape of Good Hope, where it was used by Thomas Maclear in re-surveying and extending the arc of meridian measured by Lacaille in 1750.5 The sector was returned to Greenwich in 1850 in good order, despite the difficulties of using such a long and awkward instrument 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 the Observatory’s reputation, the zenith sector was hung like a trophy on the wall of the new Transit Circle Room (Fig. 3). In the 19th century, this particular instrument had reached something approaching iconic status, even in popular accounts of the Observatory. The Religious Tract Society’s newspaper, The Weekly Visitor, described it in 1835 as ‘‘the famous zenith sector’’.6 Similarly, in 1862 Edwin Dunkin, an assistant at the Observatory, informed readers of Leisure Hour magazine that ‘‘[e]very person who claims even the slightest acquaintance with astronomy, must possess some veneration’’ for the instrument.7 Public admiration for the instrument was paralleled by appreciation of Bradley’s abilities. Indeed, the Weekly Visitor article had named him ‘‘one of the 5 Brian Warner, B. (1979). Astronomers at the Royal Observatory Cape of Good 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.
156
Lost and Found
Endeavour Vol. 38 No. 3–4
greatest astronomers of his age’’ and, half a century later, the illustrated newspaper The Graphic stated that because of ‘‘the revolution wrought in observational astronomy by his discoveries and researches’’ Newton was right to have called Bradley the ‘‘best astronomer in Europe’’.8 Despite these impressive accolades in the past, however, Bradley is largely forgotten today. The star objects at the ROG are now, undoubtedly, John Harrison’s marine timekeepers, especially since the publication of Dava Sobel’s bestselling book on Harrison, Longitude (1995). Despite the presence of his influential Graham instruments, Bradley now receives comparatively little attention. Since being highlighted on the Observatory’s audio guide a few years ago, the zenith sector, now
remounted near its working location in the Meridian Observatory, has benefitted from a little more attention, but it remains a difficult object for visitors to interpret; a half-forgotten Astronomer Royal made two confusingsounding discoveries with a very long metal tube. It is a leap to comprehend its impact on the potential precision of astronomical observation, let alone to grasp that the discovery of the aberration of light was the first observational proof of the heliocentric system. I therefore urge visitors to the Observatory to stand back (or at least, as far back as one can without bumping into Graham’s mural quadrant), to view ‘‘the famous zenith sector’’ and to recall that, even if it looks like a rusty drainpipe, ‘‘there are few instruments, even now, having such a history’’.9
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.
9 Lewis, T. (1890). ‘Notes on some historical instruments at the Royal Observatory, Greenwich’, The Observatory 163, 200–206, p. 202.
www.sciencedirect.com