A strange green beam will sweep across the Paris sky this Friday evening: why, and where to spot it?

A green laser 8.5 kilometers long in the Paris sky? That’s what Île-de-France residents can expect this Friday, October 9, 2026, as night falls. From Meudon to the capital, a scientific experiment by the Paris Observatory promises an unusually striking light show.

This Friday evening, Paris will go green! And this isn’t some alien invasion in the works. On October 9, 2026, from 19:45 to 20:45, a powerful green laser beam will cut across the sky from Meudon to Paris, spanning 8.5 kilometers. Behind this lighting show is the Observatoire de Paris - PSL, determined to put science in the spotlight for the Science Festival 2026.

The idea? A beam will depart from the Grand Dome of Meudon in Hauts-de-Seine, to reach the Arago Dome at the Paris Observatory in the 14th arrondissement, before heading back the other way. Nearly 17 kilometers round trip, covered in just 57 microseconds! Scientists will measure the travel time in real time, with the result projected on the facade of the Meudon Castle.

A green beam to turn back the clock

If the Observatory deploys a blaze of light, it isn’t just to dazzle the crowd. The exhibit marks a double milestone: 150 years since Jules Janssen founded the Meudon Observatory in 1876, and 100 years since it was joined to the Paris Observatory in 1926.

And that green beam also echoes a major discovery made 350 years earlier. In 1676, the Danish astronomer Ole Rømer demonstrated at the Paris Observatory that light travels at a finite speed. Three and a half centuries on, researchers are picking up the stopwatch again, this time with far more advanced tools.

Where to see the green laser this Friday, October 9?

No telescope is needed to enjoy the phenomenon. The beam will stay lit for an hour and will be visible from several locations:

The Meudon Castle estate and its terrace
Clamart
The Fort of Issy, in Issy-les-Moulineaux
Vanves
The surroundings of the Paris Observatory, in the 14th arrondissement

To attend the official firing, meet from 7:30 p.m. on the public terrace of Meudon Castle, at 5 Jules Janssen Square. Admission is free, but the site is not accessible to people with reduced mobility.

And for those who prefer to follow the experience from home, the readings will be streamed in real time on the Observatory’s dedicated website. Depending on the weather, additional shots may also be fired through Sunday, October 11, between 7:45 p.m. and 8:45 p.m.

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