BC Gnomonics
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Speculum Cosmographicum /
Cosmographic Mirror
The Cosmographic Mirror, though similar in appearance to a planispheric astrolabe, is a distinct instrument with distinct functions. Similarly to the pairing of terrestrial and celestial globe, this instrument allows the motion and position of the sun to be directly related to the Earth below. It folds these two Ptolemaic spheres together, creating a tool to solve cosmographic problems.
Cosmography was the study and demonstration of the relationship between the Earth and celestial sphere. While terrestrial and celestial globes emerged in the 16th century as the best tools to represent this nested system, the Cosmographic Mirror approximates a number of their benefits in a compact, cheap, easily printable educational model.
Evan Boxer-Cook
History of Science Museum
Katie Taylor
Internet Archive
How to use
(Find the subsolar point and local time around the world)
1. Set the longitude of your location (on the stereographic projection disc) to noon.
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2. Set the rule to the sun's position in the ecliptic.
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3. Rotating the rete and rule together, set the rule to your local time.
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4. Set the hour circle's indicator to coincide with the rule's position.
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5. Wherever the rule and rete intersect, the sun sits 90º overhead. Local time for all meridians can be read from the hour circle.