As night falls, a green laser beam will depart from Meudon, cross the sky to the Arago dome in Paris, and then return to its starting point. Nearly 17 kilometers round trip, covered in 57 millionths of a second. This infinitesimal time will be measured live by the Observatory teams, and the result will be displayed on the château's facade.
This event commemorates several anniversaries: one hundred years ago, in 1926, the Paris and Meudon observatories were united under a single scientific direction. The laser beam thus becomes the link between these two historic sites. It also echoes a discovery made at the Paris Observatory 350 years ago: in 1676, Danish astronomer Ole Rømer demonstrated that light does not propagate instantaneously but at a finite speed. Finally, the event recalls that Meudon, founded by Jules Janssen in 1876, is one of the cradles of astrophysics.
The laser firing sequence will be visible on-site, on the public terrace of the château de Meudon (5, place Jules Janssen, 92190 Meudon), starting at 7:30 PM for a firing at 7:45 PM. The beam will remain lit for one hour, until 8:45 PM, and will be visible from Meudon, Clamart, the Fort d’Issy in Issy-les-Moulineaux, Vanves, and up to the vicinity of the Paris Observatory in the 14th arrondissement.
The measurements will be published in real-time online at celerite.observatoiredeparis.psl.eu. Additional firings may take place from Thursday, October 8th to Sunday, October 11th, depending on the weather. The Observatory invites the public to share their images using the hashtag #LaserMeudonParis.
To accompany the experiment, the Observatory has created an educational website explaining the speed of light, from Rømer to Einstein. The projection on the château's facade is provided by LaserWave, an association of students 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.
