ratcliff models cahiers proof

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Cahiers Franois Vite, 11-12, 2007, p. 63-82.

MODELS,METAPHORS,AND THE TRANSIT OF VENUS

IN VICTORIAN BRITAIN

Jessica RATCLIFF

RsumAu XIXe sicle, lobservatoire royal de Greenwich a connu une transformation

remarquable qui a conduit certains historiens parler de la rvolution industrielle

des sciences de lobservatoire. George Biddell Airy (astronome royal de 1835

1881) est lun des acteurs principaux de cette transformation. Au sommet de sa

carrire, il sembarque dans lune de ses entreprises les plus ambitieuses,

lexprience consistant mesurer la parallaxe solaire laide du passage de Vnus

de 1874. Cet article tudie la connexion entre la conception du programme de 1874

et les nouvelles mthodes dobservation quAiry a stimules Greenwich pendant

toute sa carrire. Au centre de son programme de formation des observateurs se

trouve un modle simulant le passage de Vnus. Il sagit, de la part dAiry, dune

extension audacieuse des mthodes standard dobservation hors du domaine de

lastronomie de position pour lequel elles avaient t dveloppes. Aprs le passage,

il apparat clairement que cette prparation a t mal conue. Une interprtation

confuse de la mtaphore de la goutte noire et une confiance mal place dans la

reprsentation offerte par le modle a men des prvisions incorrectes propos

daspects cruciaux du passage de Vnus. Le programme du modle met donc en

lumire les ambitions et les limites des innovations que lindustrie avait inspires

Airy dans le domaine des sciences de lobservatoire

Abstract

During the nineteenth century, the Royal Observatory Greenwich underwent a re-

markable transformation, which has been described by historians as like an indus-

trial revolution in observatory science. George Biddell Airy (Astronomer Royal

from 1835 to 1881) was a leading figure in this period. The effort to measure the

distance to the sun (solar parallax) using observations of the 1874 transit of Venus

was one of the most ambitious enterprises of his career. In many ways the transit

programme became a proving ground for the new methods of industrial astronomy

that had been developing over the previous half-century. In order for the plan to

succeed, multiple geographically-separated observations of a particular moment in

the transits passage would have to be made. The problem was that no one alive had

ever witnessed a transit or knew what this moment would look like. The response

was to build simulation transits of Venusmechanical modelsto be used for ex-

perimentation and training. The design of these models was driven in part by calcu-


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64 JESSICA RATCLIFF

lations and in part by the astronomical reports from previous transits of Venus in the

eighteenth century. Some of these reports evoked metaphorical descriptions of a

black drop effect at the crucial moment to be observed. When it was found that

some models also produced a black drop effect, the phenomenon came to be thought

of by Airy and his assistants as real and reliable. This belief would turn out to befalse.

1. Introduction : A Problem at the Factory

The transformation of the Royal Observatory Greenwich during the

tenure of George Biddell Airy (Astronomer Royal, 1835-1881), has at-

tracted the attention of historians such as Robert W. Smith, who described

the observatorys development in the following way:

Under Airy, the Observatorys complement and types of instru-

ments, as well as methods of management, were transformed from

those characteristic of a pre-industrial society to those of a societythat had undergone the second phase of the Industrial Revolution.

The result was that, although the Observatory was certainly notable

for its scientific results, it was perhaps even more important for the

new ways of pursuing astronomy fashioned there.107

One outstanding period in Airys management career was his leader-

ship of the British program to measure the distance to the sun using obser-

vations of the transit of Venus in 1874. Using his leverage as Britains pre-

miere government scientist, Airy managed to obtain unprecedented

amounts of public funding for a project of pure research. From an initial

parliamentary grant of 15,500, the actual costs, according to Admiralty

accusations, swelled somewhere beyond 40,000108. In many ways the

1874 transit of Venus enterprise was one of the most ambitions endeavors

of Airys career, and one in which the new ways of pursuing astronomy

were put to the test. This paper discusses the practice of one particularly

107Robert Smith (1991) A National Observatory Transformed: Greenwich

in the Nineteenth Century,Journal of the History of Astronomy 22, 5-20, at p. 5.108 In terms of purchasing power, 15,000 in 1870 equalled roughly

960,000 in 1998. Robert Twigger, Inflation: the Value of the Pound 1750-

1998. London. House of Commons. February 1999. A better gauge of the finan-

cial significance of the program may be gained from comparing it to the govern-

ments annual budgeted allocation for scientific research, which (when it began in1876) was 4,000 per year.


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MODELS,METAPHORS 65

new aspect of observation employed in the transit program: the attempt to

increase the precision and accuracy of the results by training observers on

mechanical models of the transit of Venus. These simulations were a major

part of transit operations worldwide, and each of the large government pro-grams developed their own models for research, experiment or training.

Especially in Britain, as I will explain, model training was as central to the

success of the transit plan as were the telescopes, telegraph lines, and

photoheliographs. The plan was closely related to other recent changes in

the practice of astronomical observation, especially the rise of labour divi-

sion and associated developments in standardization.

In a manufacturing setting, the concept of labour division refers to

the disassembly of artisan or craft work into relatively unskilled segments

of repetitive labour. In relation to astronomy, historians and scientists from

Simon Newcomb in 1903 to William Ashworth in 1998 have identified a

similar process within the observatory as the marker of the Victorian re-

forms in astronomy. Newcomb, director of the U.S. transit of Venus enter-prise in 1874, recalled:

We may look back on Airy as the most commanding figure in

the astronomy of our time. He owes this position not only to

his early works in mathematical astronomy, but also to his

ability as an organizer. ... he introduced the same sort of im-

provement that our times have witnessed in great manufactur-

ing establishments, where labour is so organized that unskilled

men bring about results that formerly demanded a high grade

of technical ability. He introduced production on a large scale

into astronomy.109

Along similar lines, Allan Chapman has much more recently argued that

Airy was able, like the manufacturers and engineers he so admired, to

make uniformity of product in positional astronomy a matter of course, by

replacing skill with 'regularity of procedure.110

109Smith (1991) Observatory Transformed, arty. cit. p. 13.110 Allan Chapman (1992) George Biddell Airy, F.R.S. (1801-1892): A

centenary commemoration,Notes and Records of the Royal Society of London 46,p. 103-110.


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66 JESSICA RATCLIFF

Different historians have traced the division of labour at different

points in the process of observatory work.111Most relevant to the subject of

this paper are the changes that were introduced to the practice of observa-

tion, especially those aimed at minimizing residual errors due to humanparticipation in the process of measurement. As Simon Schaffer has argued,

in seeking to standardize observers, the act of observation was destroyed

and then painstakingly rebuilt through a range of surrogates for some no-

tional direct experience.112Most notably, common observations, such as

those of the passage of a star across a wire, were now corrected for individ-

ual differences in the timing of the observation. These individual differ-

ences were referred to as an observers personality or personal equa-

tion. The discrepancies generally appeared as very small and very regular

difference in the timingsome observers were slow and some were fast

of recorded stellar observations. Such differences were removed from the

final result by applying observers personal equations to the data.

The effort to regulate observation faced new challenges in the 1874transit of Venus. Whereas standardization of the observation of a stars

passage across a wire could be achieved with the personal equation, stan-

dardization of the transit of Venus observations required not only a correc-

tion for the observers reaction time but also regulation of the observers

perceptual judgement. This was simply because the observations required

for the transit of Venus were much more complex than the average astro-

nomical observation.

The transit method of measuring the suns distance depended upon

numerous geographically-separated observations of the moment, called

contact, when the edge of the silhouette of Venus touched the limb of the

sun as the planets shadow entered or began to exit the face of the sun. Inorder for the method to succeed, it was essential that observers from differ-

ent stations across the globe made identical judgements about the moment

111 In Observatory Transformed, Smith focuses on the accountant-house

style of computation. William J. Ashworth describes Greenwich feeding the net-

work of information that sustained the British Empire. See Ashworth (1998) John

Herschel, George Airy, and the Roaming Eye of the State,History of Science 36,

152-178. Holly Rothermel has described instrumental aspects of Airys factory-

like regime at Greenwich. See Rothermel (1993) Images of the Sun: Warren De

La Rue, George Biddell Airy and Celestial Photography, British Journal for the

History of Science, 26, p. 137-169, at p. 26.

112Simon Schaffer (1988) Astronomers mark time: Discipline and the per-sonal equation, Science in Context, 2, p. 115-145, at p. 118-9.


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MODELS,METAPHORS 67

of contact. Each observer had to know exactly what to look for, and all

observers had to do so in the same way. This was already a difficult task,

but an even greater problem was that, since the last transit of Venus had

occurred over a century ago, no one then alive had ever witnessed the phe-nomenon. The only guidance available was found in the observation reports

from the previous transits of Venus in 1761 and 1769. These reports con-

tained detailed descriptions of the phenomenon of contact, and often re-

sorted to metaphorical descriptions to explain what had been seen. These

descriptions, especially the so-called black drop effect, a distortion of the

planet at contact, were to have an enormous effect on the transit pro-

grammes in 1874.

How can observers be prepared to judge the appearance of contact,

when the nature of that appearance is unknown? One widely adopted solu-

tion was to develop mechanical simulations of the passage of the planet

across the sun. These models were constructed according to what was

known about the transit (the relative sizes of the Venus and the sun, forexample), but they were also appraised and modified according to the re-

ports from 1761 and 1769.

This new interest in modeling is unprecedented in the history of as-

tronomy. The models did not depend upon new technology; especially in

the case of the British models (which were not electric), similar instruments

could have been produced in the eighteenth century as well. In fact, during

the eighteenth-century transit preparations, a training model had been pro-

posed, but apparently it was never constructed. Instead, the most famous

artificial transit of that timea blend of scene painting and orrery-like

celestial modelswas created by Benjamin Martin for use in his lectures

and public shows.113

Rather, the popularity of the model approach may fit a wider mi-

metic impulse, as Alexi Assmus and Peter Galison have called it, that

some historians have argued is evident throughout Victorian science.114

113John Bevis, one of the members of the Royal Society's Transit Commit-

tee, suggested using a mechanical simulator, in which a small artificial Venus

could be observed moving across an artificial Sun, in preparing and training ob-

servers for the transit. Council Minutes of the Royal Society, November 30, 1767.

Cited in Harry Woolf (1959) The Transits of Venus: A Study of Eighteenth-

Century Science(Princeton: Princeton Univ. Press).114Peter Galison and Alexi Assmus (1989) quoted in: S. Schaffer, Where

Experiments End: Tabletop Trials in Victorian Astronomy, in Jed Z. Bushwald(ed.), Scientific Practice: Theories and Stories of Doing Physics (1995), 257-299.


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68 JESSICA RATCLIFF

Cyclones, tidal vortices, and clouds were also modeled mechanically in the

late nineteenth century. Within astronomy, Norman Lockyers laboratory

research designed to recreate aspects of the sun in the laboratory may be

seen as a similar pursuit of useful artificial representations of nature.115

2. Edward Stone and the Black Drop Effect

Although the black drop effect is a central element of histories of the

eighteenth- and nineteenth-century transits of Venus, the status of the phe-

nomenon remains somewhat mysterious. A distortion of the silhouette of

Venus near the moments of internal contact was first seen by astronomers

in 1761 and reported by many more in 1769. In 1874 less than half of the

observers would see it, and even fewer would report it in 1882. When the

transit returned in 2004, much of the interest in seeing contacts was due to

the open question of whether or not a black drop effect would be seen

(none was). It is still unclear why the black drop effect has been at times so

prevalent and at times so elusive. Possible causes include the smearing of

light by the earths atmosphere (which may vary depending on the altitude

of the sun), telescopic refraction, the expectations of the observer, or vari-

ous combinations of the three. But whatever the true nature of the black

drop effect, expectations and preconceptions about it would play a major

role in the worldwide preparations for the 1874 transit.

In order to understand the role that the black drop effect would play

in the transits of 1874 and 1882, it is necessary to go back to Edmond Hal-

leys original plan to measure the distance to the sun using a transit of Ve-

nus. The idea came to him, he later recalled, while observing a transit of

Mercury at St. Helena in 1677. At the moment when the planet fully en-tered the sun, he reported seeing a lucid line appear clearly and suddenly

at the outer edge of the planet, an event that he reckoned could be measur-

able to a small part of a second.116Seeing this line of light, Halley real-

ized that other observers at far away places on the earth also watching this

transit would see the line of light appear at a different absolute time, and

that this difference could be used to accurately calculate solar parallax. He

thus designed a plan for the much more favourable conditions of a transit of

115Ibid.116D. Sellers (2001) The Transit of Venus: The Quest to Find the True Dis-

tance of the Sun (Leeds: Maga Velda Press), p. 206. Sellers provides an Englishtranslation of Halleys original plan.


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MODELS,METAPHORS 69

Venus.117The aim, for each observer, was to record the time at which the

disk of Venus first fully entered the sun (recognizable by the appearance of

the line of light) and the time at which the planets disk first touched the

opposite edge of the sun (recognizable by the disappearance of the line) atthe end of the transit. But, as astronomers discovered when the next transits

arrived in 1761 and 1769, the crucial lucid line was much more elusive than

Halley had suggested. Instead, what most of the observers reported during

the contact of the edges of the sun and Venus was a distortion of Venuss

silhouette just as its edge was breaking away from (or coming into contact

with) the edge of the sun. As one observer put it, [t]he planet, instead of

appearing truly circular, resembled more the form of a bergamot pear, or, as

Governor Pigott then expressed it, looked like a ninepin.118 This phe-

nomenon, which came to be known as the black drop or ligament, se-

verely complicated the task of forming standard reports of a single, precise

time at which Venus fully entered and exited the face of the sun. Times of

contact recorded by astronomers side-by-side differed by as much asthirty seconds. In the end, the expeditions did not produce a result any-

where near the degree of accuracy that had been expected, and the unex-

pected complexity of the moment of contact was the primary reason

why.119

The black drop phenomenon came down to nineteenth-century as-

tronomers through these eighteenth-century reports. One common view

attributed the black drop effect to the imperfect optics of the reflecting tele-

scopes used during those transits. However as the transit of 1874 ap-

proached, ideas about the black drop began to change.

The eighteenth-century reports were periodically re-evaluated in at-

tempts to extract a better result for solar parallax from the data. Astrono-mers such as J.F. Encke and Edward James Stone (1831-1897) produced

their own values of solar parallax from the eighteenth-century observations.

Stone had been chief assistant at the Royal Observatory Greenwich since

1860. In June of 1868, he published a new analysis of the 1769 reports.

According to Stone, a re-evaluation was called for by the growing dis-

agreement between the most accepted 1769 transit value of 8".54 (by Encke

117During a transit of Venus, the planet is much closer to the earth than Mer-

cury is during its transit.118R.A. Proctor (1874) Transits of Venus, from the First Observed A.D.1639

to the Transit of A.D.2012, p. 60.

119 For a full history of the eighteenth-century transits, see H. Woolf, TheTransits of Venus, op. cit.


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70 JESSICA RATCLIFF

in 1824), and the recent results of lunar theory, speed of light measures, and

Mars in opposition, all of which pointed to a parallax of at least 8".9. De-

spite this growing disparity, Stones confidence in the soundness of the

transit method remained strong and he sought to discover a source of errorthat would explain why Enckes value was out of line with the more recent

measures.120

In his review of Enckes (and others) work, Stone found what he

called errors of interpretation. Each observation of contact typically con-

sisted of numerous times, such as Father Sajnovicss recording of contac-

tus dubius certus at 15h.26m.18s and contactus certus at

15h.26m.26s.121 The difficulty with the transit observations had always

been: which of these times shall be taken as contact? The contactus

dubius certus might have been too early, and the contactus certus might

have been too late. Should some time in between be taken, should one of

the two times be selected, or should the observation be rejected altogether?

Stones theory was that the black drop effect actually appeared at truecontact, and that what looked like geometric contact (when the edges of

the sun and Venus appear to intersect) was actually after the edge of the

planets silhouette had passed the edge of the sun. It is less important that

Stone was arguing that the black drop occurred at true contact (and later

Stones theory would be reversed) ; more importantly, the black drop was

now considered a legitimate observable phase of any transit of Venus.

With this new perspective, Stone selected observations in which it

was clear that the observer had recorded either geometric contact or true

contact, as in the formation or break of the drop. He found that the differ-

ence between observations of geometric contact versus the formation or

break of the drop was about sixteen seconds, and he thus calibrated theobservations accordingly, so that all observations, in effect, referred to the

formation or break of the drop. In essence, the black drop became the new

lucid line by which the timings of the contact observations were aligned.

On this basis, he succeeded, by simply interpreting strictly the language

employed by the observers, in producing a new parallax value of 8.91"

from the eighteenth-century data. 122The result itself did not have a wide

impact on the contemporary value of solar parallax, but because Stones

approach produced a more agreeable value of parallax, his analysis of the

120E.J. Stone (1868) A Rediscussion of the Observations of the Transit of

Venus,MNRAS, 28, p. 255, at p. 255.

121Ibid., p. 256.122Ibid.


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MODELS,METAPHORS 71

black drops features seemed to be valid, and the black drop as a real phe-

nomenon seemed to be validated as well. The gold medal of the Royal As-

tronomical Society was awarded to him in 1869 for this work.

2.1. The Transit of Mercury in 1868

These theoretical investigations of the contact phenomena were sup-

plemented by experiment and observation. For the latter, the transit of Mer-

cury in 1868 brought a chance to make actual observations of contacts of a

very similar kind (though the silhouette of Mercury is much smaller than

that of Venus). By 1868, growing interest in the transit of Venus brought,

as one observer reported, every possessor of a telescope in England out

to observe the transit of Mercury on the morning of November 5th.123At

Greenwich, six observers were stationed to observe the planets exit

(egress). In a trial run, the observers were to time the moment of contact as

if observing the transit of Venus. The results were not encouraging; time

recordings of internal contact by observers side-by-side differed by as

much as eleven seconds. Each observer saw various kinds of distortions,

though some of them were very mild.

Of the twenty-one observations of the Mercury transit published in

the MNRAS, nine reported a distortion, three reported no distortion, and

nine could not say due to boiling of the suns limb or other atmospheric

conditions. Taken all together, the 1868 observation reports formed a mass

of contradiction.

In his analysis of the observations made at Greenwich, Stone inter-

preted the reports in the same way he had interpreted the 1769 reports. Re-

garding the eleven-second disparity in contact times across observers,Stone offered practical hints for improving the results in 1874. Most im-

portantly, similar instruments with the same aperture and magnification

should be used, and attention should be directed to observations of real

internal contact [formation or break of the drop] as the chief points.

Airy, following Stone, believed that the inconsistencies in the obser-

vations could be reduced. The American transit of Venus commission on

the other hand took the results of the 1868 transit of Mercury as a major

reason to seek alternative methods. A large majority [of astronomers] will

agree, wrote Simon Newcomb in 1872, that [the method of contact] can-

not be safely depended upon until some method is found to guard against

123The reports are reprinted in the December 1868 issue of theMNRAS.


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the errors to which experience shows it to be subject.124Thus the Ameri-

can Commission began to focus on photography as an alternative. Airy, in

contrast, and although he also began a photographic wing of the pro-

gramme, remained confident in the traditional contact method.

3. The Model Solution

In March of 1873, Warren De La Rue, the chief photographer of the

British programme, wrote to Airy about making some experiments to imi-

tate the appearance of Venus on the suns disc, that is, to make photographs

of the Sun with an Artificial Venus interposed.125Airy replied that De La

Rue was not alone in starting to think about modeling the transit; the Rus-

sians and the Germans had already created a working model, and Green-

wich was having one constructed. The idea of creating an artificial transit

came from all quarters. By the end of 1873 all countries involved would

have constructed their own artificial transits.

To some contemporary journalists, the idea of astronomical modeling

was inspired by military strategy. In the London Graphicsarticle devoted

to the Sham Transit of Venus, the author explained how the Astronomer

Royal, acting on the principle which induces naval and military command-

ers to organise sham fights as a preparation for real battle, constructed an

artificial model of Venus.126This may indeed have been how the head of

instruction for the transit programme, George Lyon Tupman (1838-1922),

would have characterised the model training. Tupman, then a Captain in the

Royal Marine Artillery, had had a long career in the Navy. He became in-

volved in all aspects of the programmes management. After Airy, Tupman

was the most central figure in the British programme.The design of the British model originated at the Imperial Observa-

tory of Russia at Pulkowa.127 The British, Russian, and German models

were designed to be viewed through a telescope from a distance of 400 to

500 feet. In contrast, the French and American models were designed to be

viewed from a distance of nearly a kilometer. The long distance was in

124 S. Newcomb and G.W. Hill (1872) Papers Relating to the Transit of Ve-

nus in 1874(Transit of Venus Commission) p. 14.

125W. De La Rue to G.B. Airy (1873-03-17), CUL RGO/6-271 no. 219.126Anonyme (1874) Artificial Transit of Venus at the Royal Observatory,

The Graphic(London), 17 December.127A. Auwers to G.B. Airy (1872-08-31), CUL RGO/6-271 no. 894.


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MODELS,METAPHORS 73

order to allow for more pronounced atmospheric effects, which were be-

lieved by some to be an important factor in producing the black drop ef-

fect.128

A first version of the model at Greenwich was erected in October of1873. The event was publicised by a stack of penny-postcards sent out with

an invitation to view the model exhibiting the phnomena in their true

angular magnitude and velocity.129Viewing was open to anyone on any

weekday before 2 oclock, and a partial list of interested visitors includes

not only professional and private astronomers but also statesmen, admirals,

civil servants, and other scientists. The working of the model is clear from

figure 1. Venus is represented by a blackened metal disk that is attached to

a slider bar below. This bar is attached to a clockwork mechanism, which

pulls it along past the triangular opening in the frame. The metal Venus

advances very slowly across the opening, less than 1/1000 of an inch per

second. The opening represents the sun, with the two legs of the triangle

representing the portions of the suns edge across which the planet passes.Natural light was reflected through this opening from a rotatable mirror

positioned behind the apparatus, as seen in figure 2.

3.1. Artificial Black Drop Experiments

Just before the artificial moment of contact, a black drop effect was

seen. As Airy described it, there is disturbance of the shape, somewhat

similar to Captain Cooke's [sic.], but very much less.130This was an im-

portant turning point in the programme. The central but previously inacces-

sible black drop effect had now become reproducible and thus measurable.

The model experiments were geared more to understanding the char-acteristics of the black drop than to unravelling its cause. The first experi-

ments were conducted by George Forbes (1849-1936), a recent graduate of

St. Catherines, Cambridge, and the newly appointed professor of natural

128 The American model is described in S. Newcomb (1910) Popular As-

tronomy (2nd edn.). The French models are described in C. Wolf and C. Andr

(1876) Recherches sur les apparences singulires qui ont souvent accompagns

l'observation des contacts de Mercure et de Vnus avec le bord du soleil,Recueil

de mmoires, rapports et documents relatifs a l'observation du passage de Vnus

sur le soleil, Tome 1, partie 2.129Unsigned, (October 1873), CUL RGO/6-277 no. 632.

130Cook observed the transit (and a black drop effect) from Tahiti in 1769.G.B. Airy to G.L. Tupman (1873-11-19), CUL RGO/6-270 no. 126.


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74 JESSICA RATCLIFF

philosophy at Andersons College, Glasgow. The main purpose of Forbess

model experiments was to measure the time difference between true geo-

metric contactwhen the edge of the model Venus touched the edge of

the model sunand when contact appears to an observer to have hap-pened.131Stone had estimated from the 1769 reports that the interval was

about nine seconds. In the model, it was found to be closer to three sec-

onds. Stone had also argued that the black drop occurred at real contact,

and that geometric contact came after real contact. This idea was contra-

dicted by the model.

Further experimentation showed that this interval of time between

black drop and true contact varied with the intensity of light.132The model

used reflected sky light to represent the sun, and

by using alternately full sunlight and ordinary cloud light, the

black ligament may be seen to appear and disappearwith full

sunlight the ligament is still visible eight or ten seconds aftertrue contact, but with dull light it disappears at the third sec-

ond after contact; it is therefore expected that any atmospheric

cause which diminishes the Suns brightness will affect the

formation of the ligament.133

In general the cause of the black drop was explained with reference

to irradiation. In Forbess subsequent article series in Nature, he de-

scribed irradiation as that curious phaenomenon in virtue of which a star,

or any bright object, appears larger than it really is. If, for example a thin

platinum wire be intensely heated, it seems to a person distant about fifty

feet to be as thick as a pencil.134Very bright light was needed to produce

irradiation, and indeed the brighter the light, the stronger the black dropphenomenon appeared to be. But under dimmer circumstances, Forbes

maintained, there is a similar effect due to mental aberration a perfectly

definite phenomena that is capable of accurate investigation caused

131The results were written up in an unpublished pamphlet: G. Forbes, The

Appearances Presented by the Model Transit of Venus, Mounted at the Royal

Observatory, With a Cloudy Sky. (25-10-1873), CUL RGO/6-277 no. 665.132 Anonyme (1874) Preparations for Observing the Transit of Venus,

MNRAS, 34, p. 186, at p. 188.133Ibid.

134G. Forbes (1874) The Coming Transit of Venus: IV,Nature,May 14,p. 27-30, at p. 28.


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MODELS,METAPHORS 75

by a spreading of the excitement of the nerves of the retina, which gives

rise to the sensation of vision over a sensible space.

It is hard to pinpoint the meaning of the concept of irradiation at the

time. In Bradley Schaefers recent attempt to explain the effect, he alignsthe nineteenth-century concept of irradiation with the modern concept of

normal terrestrial atmospheric smearingthe diffusion of light through

Earths atmosphereand notes that this explanation was first proposed in

1770 by Lalande.135Other explanations were common however. Forbess

reference to physiology is one. Another held it was due to diffraction

through the atmosphere of Venus. A third proposed it was due to refraction

in telescope optics.

The cause of the black drop phenomenon remained vague. However

the model experiments seemed to confirm that the phenomenon was regular

and that it could be quantified.

3.3. Training the Observers

The most important function of the model was to train the observers.

There were two aspects to the training. One involved giving the observers,

some of whom had no background in astronomy, a basic familiarity with

astronomical instruments and the routine of observation. Tumpan devel-

oped a detailed set of instructions for the volunteers to followyou may

laugh at them he told Airy but some of the men will know little or noth-

ing about optical instruments when they begin.136

The other aspect of training involved the specific task of discerning

the moment of contact, and this involved the artificial black drop. A central

part of the telescopic plan was to train observers to view and record themoments of contact as uniformly as possible. This involved, most impor-

tantly, training observers to identify particular phases of the artificial black

drop. The goal was to minimise any individual differences in interpreting

the appearances at contact.

Practice sessions for the learners, as Francis Galton called them,

began just after the model was completed, in November of 1873, at which

time Airy dictated instructions for how the contact training was to proceed:

135 B.E. Schaefer (2000) The Transit of Venus and the Notorious Black

Drop,Bulletin of the American Astronomical Society, 197, p. 1383.136G.L. Tupman to G.B. Airy (1872-09-15), CUL RGO/6-270 no. 31-33.


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76 JESSICA RATCLIFF

Let all the observers by turns observe the Working Model with

all these instruments; and, when they are familiar with appear-

ances, let them practice the [no]ting of times. The signals will

usually be related to the formation of the drop and the forma-tion of the clear circle.137

Observers measured the model black drop again and again, with different

telescopes and eyepieces giving different appearances to the drop, marking

the times of contact on special worksheets. They were not being trained to

measure geometric contact but rather the moments in which the model

black drop formed and then broke. As observers gained more experience in

observing the formation and break of the drop, the disparity between differ-

ent observers timings decreased. This proved to Tupman and Airy that the

model was a success. The benefits of observers becoming familiar were

significant and measurable:

From the observations with the Model Transit of Venus made

at Greenwich, the following facts appear. One. It requires con-

siderable experience for an observer to appreciate all the defi-

nite changes of appearance which occur. Two. When two ob-

servers describe a particular phase which they see, and deter-

mine to observe this phase together, the times recorded by

each are generally accordant within a fraction of a second. 138

It should be emphasised here that, because of the way in which the parallax

was to be computed from the observations made, obtaining correspondence

among observers on the timing of an event during contact was the essential

thingit did not matter what event was singled out and timed by all theobservers, just that everyone singled out the sameevent. Thus the impor-

tance of the fact that observers could determine to observe this phase to-

gether with the aid of the model.

Once its power as a tool for producing consistency in observer

judgement was demonstrated, all model activity was stepped up. On

Forbess suggestion, four more models were made in London to be sent out

with the expeditions.139In India and Australia, versions of the same design

137 F. Galton to G.B. Airy (15-11-1873), CUL RGO/6-277 no. 680; G.B.

Airy to S.o.t. Admiralty (1873-03-21), CUL RGO/6-267 no. 95-104.

138G. Forbes (1874) The Coming Transit of Venus: IV, art. cit.139G.B. Airy to G. Forbes (1-11-1873), CUL RGO/6-277 no. 678.


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MODELS,METAPHORS 77

were locally produced. Before heading off to Mauritius, David Gill, the

head of Lindsays expedition, visited Greenwich in order to observe

[Airys] beautiful Model Transit of Venus, so that we may agree in ob-

serving precisely the same phenomena.140

4. December 8, 1874: veiled in a mist of words

The day arrived for the model to be set aside, and everyone took their

places for the real event. What the observers said and wrote during and

immediately after the transit has been preserved and published in two pub-

lications of the British results141

. These texts constituted the data gathered

from the telescopic observations, the descriptive language being crucially

important to how the interwoven numeric data were interpreted. The impact

of the model training is clear from these observer reports, in which frequent

comparisons of the present conditions to memories of the model are made.

For example, Tupman, chief of the Hawaiian station, reported that:

Up to [about five minutes before internal contact] the circumstances

of the Ingress of Venus exactly resembled those seen in the model. I

drew out the micrometer at 20h 45m 43s, laid it on the shelf, and in-

serted the negative eye-piece power 150, down to the pencil mark on

it for focus. This I had repeatedly practised on the model, and always

effected in 10 or 12 seconds. I was no longer than usual on this occa-

sion. On looking in I saw the cusps separated such a distance that I

thought it still wanted 30 seconds of contact, but the image not being

perfectly sharp I threw it out of focus with the rack motion, and

brought it carefully in again. As I did so I perceived that the cuspswere united by a narrow band or thread of light of sensible width,

but faint, and instantly called contact though fearing I had missed

it while focussing. I could not understand how it could have oc-

curred so much sooner than I had expected. ... I was surprised that

the band of light did not change much in appearance for some time;

140D. Gill to G.B. Airy (1874-06-06), CUL RGO/6-271 no. 385.141Airy (1881)Account of the Observations; Airy (1877)Report on the tele-

scopic observations of the transit of Venus, 1874, made in the Expedition of theBritish government, and on the conclusion derived from those observations.


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78 JESSICA RATCLIFF

it seemed a long time in comparison with the model experience.

There was no black drop nor ligament.142

Astonished and rattled, Tupman believed he somehow missed thecrucial instant moment which never happened, as he described it later in

the report. Contact happened faster than it had in the model, but at the same

time the appearances of the phenomena changed more slowly (impercepti-

bly) than in the model. He saw no black drop effect. The other observer at

Honolulu, Lieutenant Noble, reported There was no black dropno liga-

ment, but a rough dark shade which gradually faded off to a thin tint corre-

sponding to the phenomena I had observed in the model. This, instead of

being nearly instantaneous, as the model generally showed, extended over

some 20 seconds.

Tupman and Nobles reports are typical of the rest of the observa-

tionsmany comparisons to the model are made, and the general impres-

sion is of very slow, very gradual change, which did not correspond to theblack drop effect they had come to expect from the model practice. The

clarity of contact was not, however, received as good news. The hope had

been to get observers to agree on specific phases of the drop formation, and

yet they had not even agreed on whether a drop effect existed at all. As

Tupman told Airy in late July of 1875 It is time the term Black Drop was

abandoned.143

As the general consensus grew that none of the skilled observers,

who, in 1874, observed the internal contacts with good instruments, saw the

so-called black-drop phenomenon, Edward Stone defended his previous

work with a simple but fundamental point: black drop had always been

merely a metaphor for some kind of slowly changing events at contact, and

as such it was irrelevant whether or not observers literally reported a black

drop:

Nothing whatever depends upon the phrase black drop or

black drop phenomenon. This is merely the way in which

one observer, in 1769, thought proper to describe the lingering

nature of the contact, which is the cause of the only systematic

error to be feared. Whether the observer prefers to speak of

the disturbance of the apparent limb of the Sun near the point

of contact, as an ombre, a black-drop, a ligament a

142Airy (1881)Account of the Observations, op. cit., p. 269.143G.L. Tupman to G.B. Airy (1875-07-31), CUL RGO/6-272 no. 660.


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MODELS,METAPHORS 79

thread or merely to assert generally that the contact was

gradually established is a point of very little importance. The

important point...is the sensible time over which the contact

extends144

During the preparations for the expeditions, the black drop had

been transformed from an eighteenth-century observer's description into an

apparently real and definite phenomenon; it had become, as Henry Russell

of the Sydney Observatory put it, a dependable phenomenon.145Accord-

ing to Stone, observers who saw no black drop had been misled by their

preconceived ideas of the nature of the phenomenon, not because they

had actually observed something very different from those who did see a

black drop.

But of course the model training had intentionally built up precon-

ceived ideas about the contacts; that had been, in fact, a primary purpose of

the training. The British observers hadnt only expected the black drop,they had been relyingon it.

What mattered, Stone stressed, was that all observers saw the phe-

nomenon of contact occur over a certain period of time, and what must be

focused on is the analysis of that period of time, however it is described.

The essential facts, Stone concluded, are in this case, as in too many

other cases of the kind, becoming veiled in a mist of words.146

To Tupman, who was in charge of reducing the observations, that

mist seemed impenetrable. The problem, Tupman told Airy in June of 1877

as he faced the task of producing a report for Parliament, was that any

translation of the language employed would give a result ranging within

the limits of 8".65 8".85. This range was even wider than the currently

accepted values for parallax.147In search of some kind of rigor to apply to

the interpretative work, Tupman defined a system based on sets of phases

of contact that he tried to identify in each description, and aligned the

reports according to these phases. Tupman could thus ignore to an extent

144E.J. Stone (1876) On Some Phenomena of the Internal Contacts Com-

mon to the Transits of Venus, Observed in 1769 and 1874, and Some Remarks

Thereon,MNRAS, 37, p. 45.145H.C. Russell (1892) Observations of the Transit of Venus, 9 December,

1874 : Made at Stations in New South Wales, at p. xii.

146E.J. Stone (1876) On Some Phenomena, art. cit., p. 45.147G. Tupman to G. B. Airy (1877-06-09), CUL RGO/6-270 no. 404.


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80 JESSICA RATCLIFF

the language of the observers and focus instead, as Stone had suggested, on

the intervals of time between the successive timings given in each report.

Still, it was an uncomfortably subjective process. It was generally

known at the time that a reasonable value for the solar parallax lay some-where between 8".70 and 8".9, with most methods converging around 8".8.

A target good result was therefore evident, and could be pursued through

different interpretations. How should Tupman conduct his analysis in such

a way that got acceptable results but did not do so in a way that would be

considered unacceptable or manipulative? The case of a problem with the

Rodriguez data was especially tricky. After struggling unsuccessfully to

bring the observations into alignment with the phases, Airy finally in-

structed Tupman:

Try whether you canforce the Rodriguez Egresses into agree-

ment, (it can hardly be doubted that they can with fairness be

made to agree). I will take care to explain things at the end thatthere can be no imputation of coaxing.148

Throughout the painful process of reduction, Airy maintained high hopes

for Tupmans phases method. Tupman on the other hand was very pessi-

mistic about how the results would be received.149

The first official report, published in 1877 gave a parallax value of

8.760 0.122.150 The probable error, still spanning the entire range of

currently accepted parallax values, was far larger that had been hoped. A

few months later, Stone reviewed the data and returned with even more

discouraging results. Disagreeing with Tupman on many points of transla-

tion, Stone presented a parallax of 8.884 +/- 0.123.151In the end the re-

sults of the 1874 transit did not provide the conclusive result for the valueof the suns distance that had been expected.

148 G.B. Airy to G. Tupman (1877-06-11), CUL RGO/6-277 no. 405-6.

Airys emphasis.149See especially G.L. Tupman to G.B. Airy (1877-06-6), CUL RGO/6-272

no. 415-6.150 Airy (1877)Report on the Telescopic Observations, op. cit.151Airy (1877)Report on the Telescopic Observations, op. cit.; also reported

and discussed in Airy (1877) On the Mean Solar Parallax from the Transit of

1874, MNRAS, 38, p. 11.; Stone (1878) On the telescopic observations of the

transit of Venus 1874, made in the expedition of the British Government, and onthe conclusions to be deduced from these observations,MNRAS38, p. 279-295.


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MODELS,METAPHORS 81

In the second and final report of 1881, a value for solar parallax was

not given at all, ostensibly because the data from America, Britain, France,

Germany, and Russia was to be reduced together to produce the definitive

measure, but that never came about. What the 1881 report did discuss werethe histories of the expeditions and some of the problems encountered dur-

ing calculations. The misapprehension of the black drop effect was not

directly addressed, but the failure of the model to represent contacts realis-

tically was frankly described:As regards the instant of internal contact, the

appearances of the model bore no resemblance to the phenomena of the

actual transit of Venus.152

5. Conclusion

The eighteenth-century descriptions of the black drop effect and the

regular black drop-like phenomena produced by the model together led

astronomers to make erroneous predictions about the all-important moment

of contact. As the popular astronomy writer Richard Proctor would put it,

the black drop effect had attained a quasi-mythical status during the

preparations for the transit in 1874.153And this, in turn, contributed to one

of the most extraordinary experimental failures in the history of Victorian

Greenwich. Most importantly, the descriptive language in the observation

reportsupon which the interpretation of the numerical data so often de-

pendedremained a major source of uncertainty. As this history thus illus-

trates, language was one (sometimes crucial) aspect of observational as-

tronomy that was not successfully reformed during the so-called industrial

revolution in observational astronomy during the nineteenth century.

During preparations for the transit of 1874, it was accepted withoutdebate that reliable and useful inferences could be drawn from model repre-

sentations of the transit. In retrospect, that confidence seems remarkable.

How could so much faith have been placed in a simple mechanical repre-

sentation of such a complex natural event? It was not necessarily so. For

example, Norman Lockyers artificial reproduction of solar phenomena in

the early 1870s generated intense debate within the astronomical commu-

nity over the utility and scope of inferences drawn from such artificial con-

structions of celestial objects.154 Part of the difference in the case of the

152Airy (1881)Account of the Observations, op. cit., p. 56.

153This appeared in the third edition of Proctors Transits of Venus (1881).154S. Schaffer, Where Experiments End, art. cit.


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82 JESSICA RATCLIFF

transit may be explained by the time pressure under which the transit prepa-

rations proceeded; the model was a practical solution to a pressing problem.

But that decision may also have been supported by an overconfidence in

the new ways of pursuing astronomythe industrial reform of the laborsystem together with the standardization of many aspects of observation

that had otherwise been so successful at Greenwich. Further research into

the early use of modeling in science may help us to better understand the

development of what has since become a ubiquitous methodology in the

experimental and predictive sciences.

Cornell university,

[email protected]

ratcliff models cahiers proof

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