(Photos: Columbia University)
This unusual antique is called a diptych dial—a type of sundial. Skilled craftsmen produced them from the Sixteenth through the Eighteenth Centuries, usually in Nuremberg. They were used to determine the time using the light of the sun or the moon.
When opened, the string gnomon—the raised device that creates a shadow—is drawn tight. On some more sophisticated models, the string can be adjusted to account for different latitudes. Typically the top section would be used to determine the time using sunlight and the bottom section would be used to learn the time with moonlight.
This particular diptych dial dates to 1603 and is owned by Columbia University in New York City.
-via Erik Kwakkel