How did a huge project try to map the sky without a computer?

Recently, the European Space Agency released the third part 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 longest-standing questions in astronomy: How do stars (and nebulae) spread across the sky? How many are there, how far are they and how bright are they? Do they vary in position or brightness? Are there new classes of objects unknown to science?

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

Now imagine the emergence of a new technique that offers some of the benefits of the technology that makes Gaia catalogs possible. You can automatically and impartially record what you see and everyone can use it.

This 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 complete photographic survey 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 capacity to analyze them. And as the project continued, new tools made it possible to study physical processes in distant celestial bodies and distracted scientists from research, offering the chance to create new models to explain the world.

For 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 had an intuition that only photographic techniques could be useful for mapping the world. They were right, but it took more of a century and the entire careers of many astronomers for their intuition to come to fruition.

photography and astronomy

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

For centuries, astronomers struggled to record what they saw in the night sky with notes and hand-drawn sketches. It wasn’t always easy to draw what you could see by looking at the warped optics of the early instruments. You can “observe” things that are not there; The canals and vegetation on Mars, drawn by poor Schiaparelli from his Milan observatory, were nothing more than an optical illusion partly caused by the turbulent atmosphere. Only a few very well-trained astronomers like Caroline and William Herschel can instantly detect a new star in a familiar galaxy – a sign of a distant cataclysmic event?

Photography can change all that. Arago instantly recognized the enormous potential of this technique: Images captured in the depths of the night could be comfortably and quantitatively analyzed in daylight. Measurements can be precise and can be checked repeatedly.

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

Astronomers had to wait 40 years until the 1880s for very precise dry photographic plates to finally become available.