120-year-old astronomy photo plates found in Niels Bohr Institute basement

An emeritus astronomer at the University of Copenhagen’s Niels Bohr Institute has discovered a collection of photographic plates capturing astronomical images from 120 years ago in the basement. Holger Pedersen is retired now, but he was employed at the Niels Bohr Institute for many years and still visits regularly to work on personal projects. He was in the basement getting a cup of tea when he noticed some boxes marked Østervold Observatory, a Copenhagen observatory that was in use from 1861 through the 1950s. The boxes had been moved to the NBI after its closure.

Pedersen looked inside and found they contained a stacks of smaller rectangular and square containers. Curious, he took them upstairs to explore the contents. When he saw there were more than 150 carefully labeled photographic plates from as early as 1895 in the cartons, he realized he’d stumbled on an archive of, as he calls it, “astronomy archaeology.”

One box contains plates from 1895 to 1897. They are untouched and have lain there untouched ever since they were put in the box. Here is Jupiter from 1896. “It’s really the disk of Jupiter, exclaims Holger Pedersen – “how beautiful”. There are also quite a few photographs of binary stars. From the records he can see that they were taken by Carl Burrau, who worked at the Østervold Observatory. He had a doctorate in experiments with astronomical measuring instruments for photographic plates and both Holger Pedersen and several other astronomers who are following along are amazed at how good the 120-year-old photographs are. […]

Some of the photographs are from the very first years, including a lunar eclipse from 28 February 1896. A collection from glass plates from 1909-1922 show the Moon in various phases. […]

There is also a plate from 1912 that shows the massive star Arcturus in the constellation Boötes and an image shows the Orion Nebula in the constellation Orion. Some plates from 1921 show the massive star Deneb, which is a bright star in the constellation Cygnus. An image from 26 April 1957 shows the comet Arend-Roland, where you can see its antitail. An article in Nature from 1957 by Fred Whipple describes the comet’s antitail.

One of the boxes from 1941 was too heavy to hold glass plates. It turned out to contain the brass template that held the glass plates in place on front of the lens of the telescope. The telescope in question was installed in the Østervold Observatory in 1895. It was a 300 mm double reflector with a focal depth of 4.9 m and a 200 mm photographic lens with a focal depth of 4.8 m. This was cutting edge technology at the time that allowed astronomers to photograph their observations while actually observing rather than having to stop what they were doing to get the shot.

It also produced a more accurate capture, an important advancement for astronomical photography. Just a decade earlier it was still common for observatories to hire artists to create detailed images of observed phenomena because their drawings and color chromolithographs were far more detailed and sharp than photographs. The telescope is still in the dome of the decomissioned observatory today.

One of the more important plates in the collection from a history of science perspective was not actually captured at the observatory. It’s not even the original plate, in fact, but a copy of a solar eclipse captured by English astronomer Arthur Stanley Eddington on May 29th, 1919. Eddington’s observation of that eclipse is one of the most significant physics experiments of the 20th century because it tested Einstein’s General Theory of Relativity, then just 4 years old.

Eddington was one of the first astronomers in England to understand and embrace Einstein’s theory. It helped that as a Quaker and conscientious objector he wasn’t saddled with the anti-German fervor so widespread in World War I England. He and Astronomer Royal Sir Frank Dyson arranged in 1917 for two expeditions, one to Sorbal, Brazil, and one to Principe, off the west coast of Africa, to observe the 1919 solar eclipse and record whether light was bent by the gravity of the sun. Eddington’s team took pictures of the stars around the sun when the moon moved in front of it and blocked its light. The eclipse images were then compared with earlier non-eclipse views of the same stars. Eddington and Dyson found that the gravity of the sun did indeed bend the light from nearby stars, just as General Relativity predicted.

The press release from the Niels Bohr Institute says Eddington recorded the image in its collection on his trip to Sorbal, but Eddington went to Principe, not Brazil. If this is a Sorbal photo, then one of Eddington’s colleagues — Charles Davidson or Andrew Crommelin, both assistant astronomers at the Royal Greenwich Observatory — took it, not Eddington. It’s a signficant distinction because the Sorbal observations were considered to be evidence against General Relativity because they fell more in line with the Newtonian model. Eddington and Dyson discounted them because of a serious astigmatism in the telescope lens and heat of the sun on the mirror having caused severely blurred images.

The unreliability of the Sorbal setup was accepted by the astronomical community at the time, but for years afterwards Eddington’s dismissal of the Sorbal data was seen by some as a result of his bias in favor of General Relativity. The idea that Eddington fudged data to get the results he wanted still gets bruited about today even though 1979 re-analysis of the Sorbal observations confirmed that Eddington had been correct.

For more about the 1919 Eclipse Expedition, its momentous discovery and the ensuing controversy, read this genuinely fascinating paper (pdf) by Daniel Kennefick presented in the proceedings of the 7th Conference on the History of General Relativity. Dyson, Eddington and Davidson’s original 1920 paper can be read here (pdf). I highly recommend it. The language is eminently readable right at the cusp of the era when theoretical physics became all but impenetrable for a lay audience.

Eddington and Dyson’s observations of the solar eclipse of 1919 made Einstein the household name he is today. Before 1919, Einstein’s name had never appeared in the New York Times. After 1919, Einstein was a regular in the NYT and other major papers and his name became a synonym for genius.

Holger Pedersen has catalogued the collection of plates. He is hoping to secure funding that would allow him to digitize all the images for the Natural History Museum of Denmark.