Spectroscopic observations of the 1874 transit of Venus in monochromatic light.
By Peter Abrahams,
The transit of a planet across the sun is a spectacular sight through a Hydrogen alpha filter. In June of 2004, Venus will be seen as it approaches the sun, in outline against prominences & the corona.
Monochromatic observations of the sun began with Jules Janssen and Norman Lockyer, who were independently observing solar prominences in the late 1860's, using modified high dispersion spectroscopes.
Janssen published 'The First Observation of a Solar Prominence Without an Eclipse' in vol. 67 of the Comptes Rendus, 1868, and continued to improve the instruments. Lockyer had developed a similar spectroscope for viewing prominences, publishing at the same time.
Lockyer illustrates one of his instruments in: Spectroscopic Observations of the Sun. No. II; Philosophical Transactions of the Royal Society of London 159 (1869) 425-444. The user aligned the slit with the edge of the sun, aimed the telescope at the appropriate wavelength in the spectrum, and widened the slit. The images of prominences in this paper show that the method was effective, however the images were seemingly built up from smaller fields of view. This spectroscope does not seem to have any wobbling slit or image scanning mechanism.
William Huggins wrote in 1869, "Last Saturday, February 13, I succeeded in seeing a solar prominence so as to distinguish its form. A spectroscope was used; a narrow slit was inserted after the train of prisms before the object-glass of the little telescope....The slit of the spectroscope was then widened sufficiently to admit the form of the prominence to be seen. The spectrum then became so impure that the prominence could not be distinguished. A great part of the light...was then absorbed by a piece of deep ruby glass. The prominence was then distinctly perceived..." (William Huggins. Note on a Method of Viewing the Solar Prominences without an Eclipse. Proceedings of the Royal Society of London 17 (1869) 302-303.)
Johann Zoellner, in Leipzig, was observing solar prominences by 1869. Very little of Zoellner's work has been translated from German, but his observations were reported in Richard Proctor's 'The Sun: Ruler, Fire, Light, and Life of the Planetary System' (London: Longmans, Green, and Co.,1871). Illustrations of Zoellner observations of solar prominences from 1869 are compared with the prior efforts of Lockyer & Huggins, and it is noted that Zoellner "applied the same method in a systematic manner".
Carl Frederic Fearnley of University of Christiania in Oslo made detailed, beautiful drawings of prominences in 1872 & 1873, using a 15cm refractor & spectroscope. (Cited & illustrated in: W.C. Livingston, O. Engvold, & E. Jensen. Old and New Views of Solar Prominences. Astronomy 15 (1987) 18-22.) ADS lists 20 papers by Fearnley but none seem to be on this topic.
It is unclear if Janssen, Lockyer, Huggins, Zoellner, or Fearnley observed the Venus transits of 1874 or 1882 in the light of hydrogen alpha. They had the equipment but could have been involved in another observing program. Janssen was in Japan during the 1874 transit of Venus, using his 'photographic revolver' to make sequential photos.
Italian astronomers were early leaders in spectroscopy, forming the first astrophysical society (Societa degli Spettroscopisti Italiani), and publishing the first astrophysical journal (1872, Memorie della Societa Spettroscopisti Italiani).
Pietro Tacchini of Palermo Observatory traveled to India for the 1874 transit, and wrote that with his spectroscope's "large slit, before the second contact, Venus was visible over the chromosphere".
(page 49: Luisa Pigatto & Valeria Zanini. Spectroscopic observations of the 1874 transit of Venus: the Italian party at Muddapur, eastern India. Journal of Astronomical History & Heritage 4:1 (2001) 43-58.)
Illustrations reproduced in this JAHH article show a section of Venus superimposed on the solar corona. The field of view through the widened slit is narrower than the ~1 arcmin diameter of Venus.
G.L. Tupman of England was observing prominences by 1872. (G.L. Tupman Observations of the solar prominences. Monthly Notices of the Royal Astronomical Society 33 (Dec. 1872) 105-115. Instructions 105-107; list of observations 108-115)
Tupman traveled to Hawaii with a British expedition and recorded the following:
"Observation of the Ingress of Venus, 1874, Dec. 8. Report of Captain G.L. Tupman.
At 1h I carefully adjusted the direct-vision spectroscope on the 4 1/2 inch equatorial <Thomas Cooke>....The spectroscope was made by Mr. Browning, and consisted of a direct-vision prism, of five components, two flint and three crown cemented together....the slit was open 0.0016 of an inch. The power of the combination was about 55 or 60 diameters. In the focus of the eye-lens of the small examining telescope I inserted an opaque screen with a narrow slit in it, the width of which was about 1/30th of an inch, and the length rather more than the breadth of the spectrum. The Fraunhofer line C occupied the middle of this aperture...the entire spectrum was cut off from view except the C line and a narrow strip on either side....the slit was of course placed exactly in the primary focus of the object-glass. At 2h 30m local mean time I placed the slit tangentially on the Sun's limb at the expected point of first contact. The hydrogen stratum or 'chromosphere' was well defined. There was no prominence near the point of contact, but the outer limit of the chromosphere was irregular....At 3h 14m 17.5s by the chronometer I first detected the planet entering on the chromosphere. At 3h 14m 38s the curvature was so decided I was sure that it was caused by the advancing limb of the planet. At 3h 14m 47s I found it difficult to see the red line between the Sun's limb and the planet. At 3h 14m 56s I imagined the red line was severed and contact took place. At 3h 15m 15s there was a long black division in the middle of the C line which was bright on either side, while the Sun's limb was just visible among the minute irregularities on the edge of the slit. I felt sure that the contact was passed. I then removed the spectroscope and proceeded to the 6-inch equatorial."
(p40. Account Of Observations Of The Transit Of Venus, 1874, December 8, Made Under The Authority Of The British Government. ed. George Biddell Airy. H.M.S.O., 1881. Part 1, Expedition to the Hawaiian Islands. 34 megabyte .pdf Dec. 2003 )
It is expected that other spectroscopic observations of the 1874 & 1882 transits of Venus, describing the course of the planet across the chromosphere, will be found.
Peter Abrahams
20 February 2004
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