Why will a mysterious green laser cross the sky between Meudon and Paris this Friday evening?

To celebrate the 150th anniversary of the Meudon site, the Paris-PSL Observatory will fire a green laser beam between Meudon and Paris this Friday, October 9, from 7:45 p.m. to 8:45 p.m. Presented as an “experimental demonstration of the speed of light” in a press release from the Paris police headquarters, this operation also marks the centenary of the joining of the two sites. The beam, which can be visible from the Meudon area, will pass through several towns such as Clamart or Vanves before finishing its journey at the Paris Observatory in the 14th arrondissement.

This scientific experiment echoes, according to the Paris Observatory, a discovery made in 1676, 350 years ago. The Danish scientist and astronomer Ole Rømer then demonstrated in this same observatory that light does not move instantaneously, but has a defined and limited speed. This discovery constituted an important milestone in the history of astronomy.