A Civil Sky

Evening twilight descends on McDonald Observatory and the Davis Mountains of West Texas. This depicts civil twilight, when the Sun is no more than six degrees below the horizon, and the sky is still fairly bright. Full astronomical twilight, when the Sun is between 12 and 18 degrees below the horizon, will come later, providing skies that are dark enough to allow astronomers to begin staring into the depths of the universe. [Damond Benningfield]