SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Gemma Frisius. Portrait of the Flemish astronomer Gemma Frisius (1508-55). At the age of 22, Frisius proposed the idea of a mechanical timekeeper to allow longitude to be calculated at sea. This marine chronometer was invented by the English horologist John Harrison in the 1750s. Frisius also had ideas on comets and the Copernican model for the solar system.
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