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The idea is to learn a little about the cosmos and the damage done by light pollution, and hopefully to encourage participants to contribute less unnecessary light in the evenings. The group pushes on into the chilly February night: heather fringes the footpath, and young ash trees are silhouetted ominously on the hillside to the left.

Above, there is a more obvious problem: cloud cover. “If you want to guarantee clouds, all you have to do is arrange an astronomy event,” says astronomer and college lecturer Rob Jones. “But darkness is about more than just stars,” he adds quickly.

Low clouds amplify the light pollution on the fringes of this deeply dark place. The small town of Ruthin appears as obnoxiously bright as Las Vegas Boulevard compared with the inky valleys around it. Chester lies over the horizon to the east, but glows as though aflame, its light bouncing off the clouds above.

If you want to guarantee clouds, all you have to do is arrange an astronomy event

ones begins to talk us through individual stars, using a green laser pen that he appears to shoot across the galaxy. He introduces “Rigel the blue supergiant”, the brightest star in the constellation Orion. The green lance then moves north-west to point out Betelgeuse, an ancient star on its way to supernova and also part of Orion. Jones begins to stretch our level of comprehension by explaining that the star may already be dead – it’s so distant from Earth (642.5 light years) that it could have exploded before Columbus sailed to the New World in 1492, and the light from that violent event still wouldn’t have reached us yet.

With our eyes already helpfully dilated like Ibizan clubbers, the stars – little and otherwise – twinkle-twinkle above

As human settlements have grown larger and brighter, so certain species have found it harder to orient themselves. This affectsmigratory birds and, more problematically, insects, vital pollinators and the basis of many food chains. Unlike other forms of pollution, which would take years to clean out of the environment, light is one of the easiest to fix. “You just have to switch off and that’s it,” she says. “All done.”