There is no dark side of the new United States Postal Service (USPS) stamp, really. As a matter of fact, it’s all dark until you place your finger over its black circle, and then a luminous moon is revealed. The Total Solar Eclipse Forever stamp commemorates the coming August 21 eclipse, which will cross over the entire continental United States, the first such event since 1918.
The heat-sensitive design uses thermochromic ink to mask and reveal two images of the moon, both taken by astrophysicist Fred Espenak, who has the far-out nickname of Mr. Eclipse thanks to his work predicting the astronomical events. The stamp itself was designed by art director Antonio Alcalá and will have its “First-Day-of-Issue ceremony” at the University of Wyoming Art Museum in Laramie on June 20, the summer solstice. That institution may be the only art museum to have a summer solstice architectural feature (prove me wrong, commenters): a beam of sunlight passes through a solar tube at noon and lights up a silver dollar set in the floor.
While this is the first USPS stamp to involve thermochromic ink, other countries are way ahead with their novelty postage. Last September, the Royal Mail in the UK launched an Agatha Christie–themed stamp set containing hidden details in thermochromic ink; in 2013 the Belgian Post Office released stamps celebrating the centennial of Belgium’s Royal Meteorological Institute, with thermochromic ink activating patterns that represent changing seasons. Belgium has also produced stamps that taste like chocolate, which may seem like an extreme design choice if you haven’t dived into the curiosities of stamp history. For instance, in 2006 the Austria Post created a stamp incorporating meteorite powder, and way back in 1972, tiny playable vinyl record stamps were issued in Bhutan.
On the reverse of the new USPS stamp pane is a map of the 2017 eclipse’s path. NASA has a landing page with more geographic details, so you can plan to engage with the moon’s masking of the sun not just through your snail mail, but also by witnessing the 70-mile-wide “path of totality” cast across the country.