This photo displays the M45 star cluster as it slowly passes behind the Moon. The approximately 40 shots were taken at 5 minute intervals with the Moon as a reference point. In one of the initial shots, an airplane flew in front of the Moon, adding an extra element to the synthesis. See also an animation here.
Canon EOS DIGITAL REBEL XTi, Skywatcher ED 80, 18/7/2009 2:57:49 - 6:11:44, Shutter Speed 10 sec to 1/60 sec, Aperture Value 7.5, ISO 400 to 100