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Box for prism train

In the second half of the 19th Century, astronomers became infatuated with the sun. Eclipse expeditions, in particular, became key events in the astronomical calendar. Their complex planning and execution reveal how the tools of empire were as important as the new tools of astrophysics in these projects.

Taking the sun to pieces

3-prism spectrograph
Image 1: 3-prism spectrograph by J. Hammersley, London, c. 1900 (Wh.2133). Spectrographs are a modified spectroscope capable of photographing spectra, for study at leisure.

In 1859 it was discovered that when light from a celestial object is split by prisms or slits, analysis of the resulting spectrum reveals that object’s precise chemical composition.

With a new tool, the spectroscope, astronomers could—as J. Norman Lockyer put it—“take the very sun itself to pieces.”

Lockyer soon co-discovered a new element, helium, in the spectrum of the sun’s chromosphere, and instruments like the spectrograph shown in Image 1 became essential parts of the eclipse expedition toolkit.

Imperial expeditions

Solar eclipses afforded rare opportunities to study the sun’s enigmatic corona, photosphere, and prominences. A total eclipse lasts at most seven minutes, but expeditions to study them took months to plan and execute.

Huge quantities of equipment had to be moved to locations that were remote enough to escape interference from locals, but close enough to essential infrastructure like railways and military protection.

This photograph shows an example of the tent villages that would pop up at a chosen site in the weeks before an eclipse. These temporary sites needed to house the instruments, observing team, and support staff. This particular example was constructed at Pulgaon, India, in January 1898.

Read more: Eclipse teamwork at Pulgaon, 1898

Imperial support proved vital for these expeditions. Observers often included gentlemanly amateurs, government-employed scientists, academics, and their families, all travelling and working together with the assistance of colonial offices and forces.

As Lockyer recounted to his British readers, “native” locals also had to be reckoned with, a task that sometimes required force:

“The would-be observers should, some days before the eclipse, make up their minds where they will observe. This should be as far from the haunts of man as possible, and the observers should scatter widely. The natives will crowd the observers, and their talk, and perhaps even their fears, may much interfere with observations.

'In India, in 1871, my observations would certainly have been rendered impossible by the smoke of sacrificial fires … if there had not been a strong force of military and police present to extinguish them; and in Egypt, in 1882, without the protection of the soldiers, a crowd of Egyptians would have invaded the camp.”(1)

Fixing the sun

Image 2: Clockwork coelostat, by Adam Hilger, London, c. 1900 (Wh.3197)

The instrument shown in Image 2 is called a coelostat. This ingenious device was designed to keep the sun stationary during an eclipse.

Large observing instruments were too fragile and cumbersome to follow the sun as it moved across the sky, a problem the coelostat solved. Observers could aimed fixed cameras and spectroscopes at the coelostat’s mirror, which by moving slowly in opposition to the earth’s rotation held the reflected image of the sun completely still.

This instrument was previously owned by the Royal Astronomical Society, and its box bears the marks of international travel to and from eclipses.

Read more: Forgotten Instruments From an Eclipse Expedition

Invisible labour

British colonies offered up many possible sites to imperial astronomers looking to work in the shadow of an eclipse. In this photograph, equipment is being unloaded from a vessel of the Australian Navy onto the beach near Wallal, in preparation for an eclipse observable on 21 September 1922.

A film made of this expedition, The Sun Worshippers, chronicles the work there, which included observations to test Einstein’s general theory of relativity. Notably absent in the film are the Nyangumarta people seen at work in this photograph. Local labour, almost invisible in official eclipse reports, was essential for building, stocking, and maintaining field sites.

References

1.     J. Norman Lockyer, Recent and Coming Eclipses (London: Macmillan, 1897), pp.17–18.

Joshua Nall

Joshua Nall, ‘The Sun Worshippers’, Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2020.

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