Royal Sovereign and light pollution at the Royal Greenwich Observatory

Dr Paul Murdin.

Date: April 21, 2025

In 1971, the lighting of the towns around the Royal Greenwich Observatory at Herstmonceux Castle had caused light pollution to grow and become a considerable factor inhibiting its work. The observatory began public discussion about lighting controls, including restrictions on the type and operation of domestic lighting around homes in the neighbourhood of the observatory, and illumination of evening sports facilities. These proposals would cause inconvenience and the proposals met with understandable resistance. The Royal Sovereign’s lighthouse beam figured in these discussions, along the lines of “Why am I being told not to install the kind of security lighting around my house that I want, when you let the lighthouse shine so brightly?”

Dr Alan Hunter was the Director of the Observatory at the time and responded by raising the issue with Trinity House. They engaged a scientific consultant to propose solutions. Proposals to switch the light off as it rotated past the azimuth of the observatory were vetoed on account of the increased risk to boats navigating between the lighthouse and the shore. The consultant came up with the favoured solution, namely to install an optical device on the safety railings around the light at the right azimuth, to dip the beam as the light rotated past the telescopes.

The device was of a type known as a Fresnel lens, named after Augustin-Jean Fresnel (1788 – 1827), a French civil engineer and physicist. It was an array of prisms, mounted in parallel, horizontally. The lighthouse beam was deviated by a small angle downwards as it passed through the prisms, such that the top edge of the beam skimmed along the seashore 10-15 km from the lighthouse, maintaining illumination of the inshore sea. Because of spill-over from the beam, the lighthouse was still visible from the telescopes but with a reduced intensity. It was also possible still to see the beam sweeping over the shoreline west of Eastbourne and east of Bexhill and, it seemed, passing at high altitude over Herstmonceux. The effect of the solution could have been more dramatic.

As a junior member of staff, I had been searching with the observatory’s telescopes for the flashing light from optical pulsars, which are natural, celestial lighthouses. Fearing that people would be unimpressed by the Fresnel lens solution, Hunter instructed me to measure light in the sky from the Royal Sovereign’s beam as seen from the Observatory before and after the remedy was installed to show that the Fresnel lens had the desired effect. To do so, I pointed my equipment on the 36-inch telescope at the sky above the lighthouse. I had hoped that a perk of accepting this task would be a dramatic sea-voyage out to see the lens and the lighthouse but I was disappointed.

My task was more difficult than I had anticipated. The beam had a dramatic appearance, but I had to push the telescope very low to detect even a glimmer of light from it, so low I began to think the mirror might topple out (it’s ok, the mirror wouldn’t do that but it might dislodge). The altitude below which I could detect something was much lower than one at which we would normally carry out astronomical observations. The beam width of the light was narrow and the light that scattered in its passage through air overhead was strongly directed forwards, not downwards. The beam was bright when viewed horizontally but the intensity fell off sharply with the scattering angle, and merged in with the general light pollution.

Another issue was that the intensity of the scattered light was very variable, from night to night and even hour to hour. I think this was probably because of variable atmospheric conditions – sea fog coming and going. My results were not very clear.

I reported this to Hunter. He said that it was important to use the fact that a respected organisation like Trinity House had responded to our interests, so as to set an example to the population at large. He told me not to say anything detailed about the results.

I was a junior member of staff and I could see that explaining beam angles and scattering in public meeting was going to be tricky. I decided that I would let Hunter negotiate his way through this situation on his own. I gave him graphs of the positive detections to show to people if he needed to, and he may well have done so, but I heard no more of it. However, a code of practice countering light pollution was implemented by the local building control authorities.