How a huge project tried to map the skies without computers

Recently, the European Space Agency released the third installment of data from the Gaia satellite, a public catalog that provides the positions and velocities of over 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 are there, how far away are they, and how bright are they? Do they change position or brightness? Are there new classes of objects unknown to science?

For centuries, astronomers have attempted to answer these questions, and the work has been laborious and time-consuming. It wasn’t always easy to record what you could see through your telescope lens, if you were lucky enough to have a telescope.

Now imagine the emergence of a new technique that, for the time, offered some of the advantages of the technology behind the Gaia catalogs. It could automatically and impartially record what you see, and anyone could use it.

This technique was photography.

This article chronicles how photography changed astronomy and how hundreds of astronomers formed the first international scientific collaboration to create the Carte du Ciel (literally, “Sky Map”), a comprehensive photographic study of the sky. This 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 create the largest 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 did not match our ability to analyze them. And while the project was underway, new instruments made it possible to study physical processes in distant celestial objects, attempting to distract scientists from the investigation by offering the possibility of creating new models to explain the world.

For astronomers working on the Star Chart, no model yet existed that could summarize the positions of millions of stars in a theory of the evolution of our galaxy; rather, researchers had only a hunch that photographic techniques might be useful in mapping the world. They were right, but it took nearly a century and the entire career of many astronomers for their intuition to bear fruit.

Photography and astronomy

It was the astronomer and explorer François Arago, president of the Paris Observatory, who announced Louis Daguerre’s photographic techniques to the world. Daguerre, drawing on the work of Nicéphore Niépce, discovered how to make permanent images on metal plates.

For centuries, astronomers struggled to record what they saw in the night sky with hand-drawn notes and sketches. Looking through the distorted optics of early instruments, it was not always easy to draw what one could see. You might “observe” things that weren’t there at all; those canals and vegetation on Mars that poor Schiaparelli pulled from his Milan observatory were nothing more than an optical illusion, caused in part by the turbulent atmosphere. Only a few highly trained astronomers, like Caroline and William Herschel, could instantly spot a new star in a familiar galaxy – the signal for a distant cataclysmic event?

Photography could change all that. Arago immediately understood the immense potential of this technique: images taken in the depths of the night could be comfortably and quantitatively analyzed in daylight. The measurements could be accurate and they could be checked repeatedly.

Daguerre received a pension and allowed Arago to open up the details of his procedure, which led to an explosion of portrait studios in Paris and around the world. But it turned out that Daguerre’s method just wasn’t sensitive or practical enough to capture anything other than the brightest stars, the Sun or the Moon. The next hot new technology, wet-plate collodion emulsions, wasn’t much better; the plates dried out during the long exposures needed to capture faint astronomical objects.

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