Every night, a satellite hangs over the Andes, looking down. It doesn't see stars but clouds, and it records where they gather and where the sky stays clear. For five straight years, the satellite took images, and from them emerged a map: it shows where you'd best place a telescope to see stars most often.

A telescope needs clear nights, and in the mountains, clouds behave unpredictably. In the past, to find a good site, scientists would go on expeditions and live on mountaintops for months. Now you can simply look at satellite images. The satellite measures cloud temperature: a cloud is always colder than clear sky. The researchers took records from 63 weather stations to know exactly what temperature means a cloud, and taught the satellite to tell the difference.

The map revealed twelve places where the sky is clear more than 30% of dry nights. That means a telescope placed there would see stars nearly every third night. The method was validated against observatory observation logs: the satellite didn't miss. Now any university can pick a telescope site without spending a year in the mountains.