How a major project tried to map the sky without computers

Increase / Image showing the emission nebula, Eta Carinae (formerly Eta Argus) taken by the Astronomical Telescope at the Royal Observatory, Cape of Good Hope, South Africa. At the center of this intricate nebula lies a massive but unstable star that will one day explode spectacularly.

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Recently, the European Space Agency released the third batch 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 the long-standing questions in astronomy: How do stars (and nebulae) spread across the sky? How many are there, how far away and how bright are they? Do they change location or brightness? Are there new classes of things unknown to science?

Astronomers have been trying to answer these questions for centuries, and that work has been painstaking and time-consuming. It hasn’t always been easy to record what you can see in the eyepiece of a telescope – if you’re lucky enough to own one.

Now imagine the emergence of a new technology that at one time provided some of the advantages of the technology that made the Gaia catalogs possible. It can automatically and impartially capture what you see, and anyone can use it.

That technique was photography.

This article tells how photography changed astronomy and how hundreds of astronomers formed the first international scientific collaboration to create the Carte du Sel (literally “map of the sky”), a complete photographic overview of the sky. This collaboration resulted in a century-long struggle to process thousands of photographic plates taken over decades, while the locations of millions of stars were manually measured to create the largest catalog of the night sky.

Unfortunately, the Carte du Ciel project was created at a time when our ability to collect measurements from the natural world was not compatible with our ability to analyze them. While the project was underway, new tools made it possible to study physical processes in distant celestial bodies, taking scientists away from research, offering the opportunity to create new models to explain the world.

For the astronomers working on the Carte du Ciel, there was not yet a model that could abstract the positions of millions of stars into a theory of how our galaxy evolved; Instead, the researchers just had a hunch that photographic techniques might be useful for mapping the world. They were right, but it took more than a century and the careers of many astronomers for their intuition to pay off.

Photography and astronomy

Increase / Astronomical telescope used at the Royal Observatory in Greenwich to survey the sky for photographing the Carte du Ciel. The instrument consists of two refracting telescopes placed together on an equatorial base. One was used to capture the image, while the other provided accurate tracking during the long exposures required by the low-sensitivity films then available.

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Astronomer and researcher François Arago, head of the Paris Observatory, was the one who announced Louis Daguerre’s photographic techniques to the world. Based on the work of Nicéphore Niépce, Daguerre discovered how to make permanent images on metal plates.

For centuries, astronomers struggled to record what they saw in the night sky with handwritten notes and drawings. Given the distorted optics of the old instruments, it wasn’t always easy to draw what you could see. You can “perceive” things that were not there; Those canals and vegetation on Mars that poor Schiaparelli painted from the Milan observatory were nothing more than an optical illusion, partly caused by the turbulent atmosphere. Only a few highly trained astronomers, such as Caroline and William Herschel, were immediately able to detect a new star in a known galaxy – a sign of a cataclysmic event in the distance?

Photography can change all that. Arago immediately recognized the enormous potential of this technology: images taken at night can be comfortably and quantitatively analyzed in daylight. Measurements can be accurate and can be checked frequently.

Daguerre was granted a pension and allowed Arago to reveal the details of his actions, resulting in an explosion of photography studios in Paris and around the world. But as it turned out, Daguerre’s method was neither sensitive nor practical enough to capture anything but the brightest stars, sun or moon. The next new technology, wet collodion emulsions, was not much better; The plates will dry out during the long exposures required to record faint astronomical objects.

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

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