How a huge project tried to map the skies without computers

Enlarge / Photograph showing the emission nebula, Eta Carina (formerly Eta Argus) taken using the Astrographic Telescope at the Royal Observatory, Cape of Good Hope, South Africa South. At the center of this complex nebula is a massive but unstable star that will one day explode in spectacular fashion. SSPL/Getty Images

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.

That 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, "Carte du Ciel"), 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. /p>

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

Enlarge / The astrographic telescope used at the Royal Observatory in Greenwich for the photographic survey of the sky Carte du Ciel. The instrument consists of two refractors...