This photograph was taken at night in Rochester, NH under the January "Wolf Moon." The sky is blue in this image for the same reason that it is blue during the day. Moonlight is scattered by molecules in the air. The wavelengths of light that are scattered the most are the longer ones. Thus, the majority of wavelengths that move straight through the atmosphere are the shorter ones at the blue end of the spectrum. The blue of a moonlit sky is so faint, however, that a long exposure (in this case, 3 hours) is required to reveal it. Long exposures also make star trails; slow, graceful arcs traced by the stars as Earth spins on its axis.