DAVID PARKER / SCIENCE PHOTO LIBRARY DAVID PARKER / SCIENCE PHOTO LIBRARY
Plate from 18th century encyclopedia showing an illustration of an armillary sphere (variations are known as spherical astrolabe, armilla, or armil). An armillary sphere is a model of objects in the sky (in the celestial sphere), consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic. As such, it differs from a celestial globe, which is a smooth sphere whose principal purpose is to map the constellations.
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