Star Trails over Mobius Arch. This rock formation is sometimes referred to as Galen’s Arch in honor of late mountaineer and nature photographer Galen Rowell. Alabama Hills, Owens Valley, California, USA. The picture was taken over a one-hour period, facing north so that the Earth’s rotation would be recorded as trails of stars appearing to turn around Polaris. Exposure was allowed to continue until shortly before moonrise so that the gradual lightening of the sky would reveal the distant rocks and hills as a silhouette. While the shot could have been done as a single exposure, this particular image consists of about 80 individual 45-second exposures, taken using an interval timer and stacked together digitally to keep noise to a minimum. During the last exposure, a single half power burst of flash from a 580EX II, equipped with a 1/2 CTO filter, was fired by hand. This illuminated the arch, which would otherwise have been completely black. EOS 5D Mark II, 80x 45 sec at f/4, ISO 200, 17mm.