So for more than a decade, the two towns and a local dark-sky nonprofit have been dialing down the dimmer switch. They have replaced streetlights and passed rules requiring that outdoor lights point down. The group built a small observatory with star guides who tee up its telescope and take people on a tour of the night. They coax homeowners to hood their porch lamps or dim a bright light outside their house.

“People out of ignorance go with whatever’s cheap or whatever’s brightest,” said Ed Stewart, a board member of the local dark-sky group. “You multiply that by 200, 300, and there goes the sky.”

He said advocates met with homeowners’ associations and held stargazing parties to sell the virtues of the night. When they gaze over the valley and see winking floodlights on a ranch or home in the hills, they see their next targets of persuasion.

“You can’t just go up to someone and say, you’ve got a bad light, and legislate the problem away,” he said. “People resist that, especially in Colorado.”