In 1908, in the Paris suburb of Meudon, someone began pointing an instrument at the Sun and recording what it looked like. Then they did it again the next clear day. And the next. That habit has held, with the exception of one long break, for well over a century. The regular observations started with calcium light in 1908 and hydrogen light in 1909, and they have never really stopped since.
What strikes us about this is not any single frame. It is the stubbornness. A person, then a team, then a team of successors, showing up on ordinary clear days for generations to take essentially the same picture. That is the kind of work that looks pointless up close and turns out to be valuable from a distance.
A telescope in a Paris suburb that keeps staring at the Sun
The instrument doing the staring is the Meudon spectroheliograph, designed at the observatory by Henri Deslandres and Lucien d’Azambuja. It is part of the Paris Observatory, and its daily watch on the Sun has now run for more than a century.
How often it works depends on the sky over Meudon. Malherbe and Dalmasse note that “Weather conditions in Meudon allow at least one daily observation between 250 and 300 days per year.” So the record is not literally every day. It is every day the weather permits, which over a century adds up to more than 120,000 images on glass plates, film and, later, digital cameras.
What a full image of the Sun actually captures
These pictures are not snapshots the way a phone camera makes them. The instrument scans across the whole face of the Sun through a narrow slit, capturing just one color of light, and builds up a full picture as the image drifts past. At Meudon the whole disk is scanned in about 30 seconds.
The choice of color matters. Calcium light and hydrogen light each reveal a different layer of the Sun’s atmosphere, so the same star looks like a different object depending on which one you record. Day after day of that, in the same colors, gives you something a one-off image never can: a way to watch the Sun’s face change slowly, the way a time-lapse shows what the eye misses in real time. The value is in the sameness. Every plate taken the same way is a frame you can line up against every other frame.
The one gap in the record: the First World War
There is exactly one long hole in this series, and it is a grim one. Malherbe writes that “Observations were totally interrupted during WW1.” Deslandres and d’Azambuja were called up to serve, and the daily observing did not restart until 1919.
The contrast that stays with us is the Second World War, which the series survived. The observations continued through it despite a shortage of hands. Only the first war was enough to stop the Sun being photographed at Meudon. For a project defined by continuity, that single interruption shows how deliberately the rest of it was kept going.
From glass plates to a public archive anyone can open
The technology underneath the habit kept changing while the habit stayed the same. Glass plates gave way to film, and in 2002 digital cameras replaced the photographic ones. The old plates and films did not become useless. They became a backlog to be rescued.
One person has been unusually central to that rescue. Isabelle Bualé joined the Paris Observatory in 1985 and went on to head its solar observation service. The Observatory credits her with setting up the complete digitization of the collection: over twenty years, more than 80,000 photographic plates and 130 kilometers of 35 mm film were digitized. It is an oddly physical way to think about a digital archive. Before those images could become files in a database, there were tens of thousands of pieces of glass and a strip of film long enough to stretch well beyond Paris.
And the job is still not simply finished once an image has been scanned. A second, sharper pass through the plates was underway when Malherbe described it: “A second digitization of plates is in course, with much better resolution… but it is a rather slow process which requires a lot of time for operators, which are also observers.” That last phrase is the honest heart of it. The people scanning the archive are the same small group still running the daily observations, and the sharper scan was meant to finish around 2026. A handful of people, not a large operation.
All that scanning means the images do not sit in a vault. They flow into BASS2000, the Paris Observatory’s free public database, where the older plates are available month by month. A record built by hand over a century is now something you can open in a browser.
Why a century of near-daily snapshots is worth it now
This is where the patience pays off. Satellites have only been measuring the Sun directly for a few decades. To ask how the Sun behaved before that, you need a stand-in that reaches much further back, and the brightness in calcium-light images turns out to be one. Researchers have used these old calcium-light images to work out the Sun’s past magnetism and how much energy it gave off. That in turn feeds a harder question: how much the Sun shaped Earth’s climate before anyone could measure it from space.
Length matters more than sharpness here. A short record can tell you what the Sun is doing. A long one lets you ask what the Sun does over many cycles, and whether a given decade was unusual. The authors themselves make the careful version of the claim, describing the Meudon series as “probably the longest available worldwide,” .
As Malherbe puts it, “The Meudon collection, spanning 10 solar cycles, remains exceptional for long-term reconstructions of solar variability and for studies of rare events.” He is describing his own institution’s archive, so read that as a proud claim rather than a neutral verdict. But the logic behind it is sound. You cannot manufacture a hundred years of daily images after the fact. Either someone showed up on the clear days back then, or the record does not exist. Nobody in 1908 knew that calcium-light brightness would one day be read as a fingerprint of the Sun’s magnetism. They photographed the Sun anyway, and left later scientists a question they could not have asked with a shorter record: not what the Sun looks like today, but what it has been doing all along.