I've always been fascinated by eclipses. An eclipse of the Moon provides a front-row seat to watch Earth's shadow overtake another celestial body. There is a bit of drama watching for that first sign of very weak shading as the Moon glides into the pale outer fringe of the Earth's shadow. The shadow cone of the Earth has two parts: the dark, inner umbra, surrounded by the lighter penumbra. The penumbra is the pale outer portion of the shadow. It can be seen as a faint shadow slowly moving across the Moon with the unaided eye. This faint shadow can generally be detected when the Moon is about 50 to 70 percent of the way into the penumbra.

In the April 2005 Sky & Telescope, senior editor Alan MacRobert issued a challenge to determine how early the penumbral shading becomes detectable during an eclipse. I wondered how early the penumbral shadow can be detected using a digital camera and image processing software. The encroachment of the penumbral shadow during a lunar eclipse is very subtle. Digital images can capture these slight variations, and enhancement by digital processing will allow us to see them. The partial eclipse of the Moon on October 17, 2005, provided an opportunity to explore this question.