As night falls, a green laser beam will set off from Meudon, cross the sky to the Arago dome in Paris, then return to its starting point. Nearly 17 kilometres there and back, covered in 57 millionths of a second.
This tiny interval will be measured live by the Observatory’s teams, and the result will be displayed on the façade of the château.
One anniversary, three stories
In 1926, the Paris and Meudon observatories were brought together under a single scientific leadership. A hundred years later, the beam becomes the link between these two historic sites.
The event also echoes a discovery made at the Paris Observatory 350 years ago : in 1676, the Danish astronomer Ole Rømer demonstrated there that light does not travel instantaneously, but at a finite speed.
And it is a reminder that Meudon, founded by Jules Janssen in 1876, is one of the birthplaces of astrophysics.
Where and when to see it
- On site : the firing sequence is open to all on the public terrace of the Château de Meudon, at the foot of the Jules Janssen statue, 5, place Jules Janssen, 92190 Meudon.
Gather from 7.30 pm for a firing at 7.45 pm. - From afar : the beam will stay on for an hour, until 8.45 pm. It will be visible from the Meudon estate, Clamart, the Fort d’Issy in Issy-les-Moulineaux, Vanves, and as far as the surroundings of the Paris Observatory in the 14th arrondissement.
- Online : the measurements will be published in real time on celerite.observatoiredeparis.psl.eu, with the site fully live from 7 October.
Weather permitting, further firings may take place from Thursday 8 to Sunday 11 October, between 7.45 pm and 8.45 pm. To make sure you don’t miss a thing, follow the latest news on the Observatory’s Instagram account.
Film it, share it
Spotted the beam ? Film it, without flash, and share your footage with #LaserMeudonParis. The best shots will be shared by the Observatory.
Understanding the speed of light
To accompany the experiment, the Observatory has created an educational website for curious minds, pupils and teachers. It tells how a seemingly simple question (how fast does light travel ?) transformed our understanding of the Universe, from Rømer to Einstein.
The projection on the château façade is provided by LaserWave, a student association from the Institut d’Optique Graduate School. The event is part of the Fête de la Science, in partnership with Universcience and the City of Meudon.
Practical information
Friday 9 October 2026 · firing at 7.45 pm · beam visible until 8.45 pm
Public terrace of the Château de Meudon, 5, place Jules Janssen, 92190 Meudon
Free admission · outdoor event · site not accessible to people with reduced mobility
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