How a huge project tried to map the sky without computers

Enlarge / Photograph showing the emission nebula, Eta Carina (formerly Eta Argus) taken with the astrographic telescope at the Royal Observatory, Cape of Good Hope, South Africa. Located at the center of this intricate nebula is a massive but unstable star that will one day explode spectacularly.

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Recently, the European Space Agency published the third installment of data from the Gaia satellite, a public catalog that provides the positions and velocities of more than a billion stars. This is our latest attempt to answer some of astronomy’s oldest questions: How are stars (and nebulae) distributed across the sky? How many of them are there, how far away are they, and how bright are they? Do they change position or brightness? Are there new classes of objects that are unknown to science?

For centuries, astronomers have tried to answer these questions, and that work has been painstaking and slow. It wasn’t always easy to record what you could see in your telescope lens, if you were lucky enough to have a telescope.

Now imagine the rise of a new technique that, for its time, offered some of the benefits of the technology that enabled the Gaia catalogues. It could automatically and impartially record what it sees, and anyone could use it.

That technique was photography.

This article tells the story of how photography changed astronomy and how hundreds of astronomers formed the first international scientific collaboration to create the Carte du Ciel (literally, “Map of the Sky”), a comprehensive photographic survey of the sky. That collaboration resulted in a century-long struggle to process thousands of photographic plates taken over decades, with the positions of millions of stars measured by hand to make the largest ever catalog of the night sky.

Unfortunately, the Carte du Ciel project came at a time when our ability to collect measurements of the natural world was not matched by our ability to analyze them. And while the project was in progress, new instruments made it possible to study physical processes in distant celestial objects, tempting scientists away from the survey by offering the opportunity to create new models to explain the world.

For the astronomers working at the Carte du Ciel, no model yet existed that could abstract the positions of millions of stars into a theory of how our galaxy evolved; Instead, the researchers had only a hunch that photographic techniques might be useful in mapping the world. They were right, but it took almost a century and the entire career of many astronomers for their intuition to bear fruit.

photography and astronomy

Enlarge / The astrographic telescope used at the Royal Greenwich Observatory for the Carte du Ciel photographic survey of the sky. The instrument consists of two refracting telescopes mounted together on an equatorial mount. One was used to take the picture, while the other was to ensure accurate tracking during the long exposures required for the poorly light-sensitive films available at the time.

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It was the astronomer and explorer Francois Arago, president of the Paris Observatory, who introduced the world to the photographic techniques of Louis Daguerre. Daguerre, building on the work of Nicéphore Niépce, discovered how to make permanent images on metal plates.

For centuries, astronomers have struggled to record what they saw in the night sky with handwritten notes and sketches. Looking through the distorted optics of early instruments, it wasn’t always easy to draw what you could see. You could “observe” things that weren’t there at all; those channels and vegetation of Mars that poor Schiaparelli extracted from the Milanese observatory of him were nothing more than an optical illusion, caused in part by the turbulent atmosphere. Only a few highly trained astronomers such as Caroline and William Herschel were able to instantly detect a new star in a familiar galaxy – a sign of some distant cataclysmic event?

Photography could change all that. Arago instantly realized the immense potential of this technique: images taken in the depths of night could be conveniently and quantitatively analyzed in daylight. Measurements could be accurate and could be repeatedly verified.

Daguerre received a pension and allowed Arago to divulge the details of his procedure, sparking an explosion of portrait studios in Paris and around the world. But it turned out that Daguerre’s method simply wasn’t sensitive or practical enough to capture anything but the brightest stars, the Sun or the Moon. The next new technology craze, wet plate collodion emulsions, was not much better; the plates would dry out during the long exposures required to capture faint astronomical objects.

Astronomers had to wait 40 years, until the 1880s, for highly sensitive dry photographic plates to finally become available.