An astronomical compendium (plural = compendia) is an instrument that carry numerous devices for telling the time and performing astronomical calculations. Many compendia were made in the German lands in the late 16th and early 17th centuries. They are often beautifully engraved in gilt brass. Typically such compendia carry a sundial, various lunar and solar volvelles, a compass, tables of latitude, and a perpetual calendar.
Two characteristics are typical of the construction of these instruments: first, they were often made as lavishly as possible; second, they are ingeniously constructed, with as many instruments as possible filling the available space.
Most of the instruments on a compendium are used to simplify astronomical calculations. Many compendia have volvelles - rotating discs that show the phases of the Moon, the positions of planets, and other such phenomena.
Almost all compendia have at least one form of sundial. These are often adjustable for use in different places, and are accompanied by lists of the latitudes of major cities around the world. Sometimes these lists are obviously functional, including various towns and major ports, but often they are more fanciful, including places such as Babylon, Alexandria, Moscow, Cuba, Constantinople, and Nineveh (an important ancient city in Assyria). Like the gilt decoration and detailed engraving, these were intended to show the wealth and status of the instrument's owner.
Some compendia also carry stereographic projections. These are multi-purpose maps of the heavens, allowing many astronomical calculations to be simplified. Using these, people could determine the time of sunrise and sunset, and the position of the Sun in its annual (apparent) motion through the sky.
Boris Jardine
Boris Jardine, 'Astronomical compendia', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008
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