The mechanism for making the change is thermochromic ink, which by definition changes with heat. Two photos are embedded on the stamp, and the application of heat, like the body heat emitted by a finger, switches the active photo to the other option. In effect, the Sun becomes the Moon, which is what eclipse watchers see in the sky at the moment of total solar eclipse. Removal of the finger or otherwise heat source allows the stamp to revert to the initial image. On the back of the stamp is a map of the eclipse path.

The eclipse will be visible from parts of 14 states, diagonally southwest, from Oregon (beginning mid-morning West Coast Time) to South Carolina (about mid-afternoon East Coast Time). Viewing will be possible from coast to coast for the first time since 1918. The last total solar eclipse visible on the American mainland was in 1979.

The Postal Service will have a First-Day-of-Issue ceremony on the summer solstice, June 20, at the Art Museum of the University of Wyoming, in Laramie.

Astrophysicist Fred Espenak, who runs the Mr. Eclipse website, took both photos. The pre-heat photo was of a March 29, 2006, eclipse, which Espenak traveled to Jalu, Libya, to observe. The post-heat photo is of a full moon.

Preorder stamps here.