It's 12:54 a.m., middle of the night, and gasses that make the sky blue, in the sunlight, are invisible at night. I want to practice more long-shutter-speed photography. ........FULL-MOON LIGHT and light from a freeway billboard light up the landscape, but it is my long, 30-second shutter speed that allows us to see the invisible blue. It is just so incredibly blue, even at night!