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Lost and Found
Endeavour
Vol. 38 No. 3–4
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‘‘The famous zenith sector’’ at Greenwich
Rebekah Higgitt
1
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 difficult
question to answer. One can have any number of favour-
ites 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 some-
thing that can genuinely be said to have had an important
impact on the history of practical astronomy, instrument-
making 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. Converse-
ly, 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 instru-
ments 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
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
2
1
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
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.
3
www.sciencedirect.com
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 in-
strument and its maker, writing in
Philosophical Transac-
tions
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 altera-
tions, until a new instrument was brought into use in 1812
(Fig.
2).
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
4
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
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.
5
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 as-
tronomy 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, how-
ever, 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 confusing-
sounding 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
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.
www.sciencedirect.com
8
Lewis, T. (1890). ‘Notes on some historical instruments at the Royal Observatory,
Greenwich’,
The Observatory
163, 200–206, p. 202.
9

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