The Greenwich Meridian Laser

If you’re in Greenwich, East London, or even further afield after the Sun sets, you might notice a bright green beam in the sky. 

Sadly aliens aren't communicating with us: this is the Greenwich Meridian laser, beamed north from the Royal Observatory Greenwich to mark the Prime Meridian (0 degrees longitude).

Designed by artists Peter Fink and Anne Bean, the laser is titled 0 degrees. It was initially set up in 1993, which required a permit from the Civil Aviation Authority, before the laser was upgraded in 1999.

The beam is reported to have been seen up to 36 miles away with the naked eye under excellent seeing conditions, and even farther with binoculars.

It originates from a point above the Airy Transit Circle, the telescope historically used to determine the Prime Meridian.

The laser is switched on each evening when the Royal Observatory closes and switched off at 11pm. It's much harder to spot - if it's visible at all - on summer evenings, due to the late setting of the Sun. However, the laser is easily visible during the long, dark winter evenings.