The Armillary Sphere
A model of the heavens built from a nest of rings, each standing for an important circle of the sky — the celestial equator, the ecliptic, the tropics and the horizon. It shows, in three dimensions, how the sky is organised around a central Earth or Sun.
The rings
The name comes from the Latin armilla, a ring or bracelet. Each ring represents one of the great circles astronomers use to map the sky, so the sphere makes visible at a glance what a flat chart can only imply.
Two kinds
Demonstrational armillaries were teaching models, used to show how the circles of the sky relate. Observational armillaries were larger, precise instruments used to measure the actual positions of celestial bodies.
Its legacy
For centuries the armillary sphere was the emblem of astronomy itself, and it remains one of the clearest ways to understand the geometry the astrolabe flattens onto a disc.
Connections
- The Astrolabe — An ancient instrument for measuring the altitude of stars and planets and for solving problems of time and position in the sky. Often called the first analogue computer, it let a single hand-held disc tell the time, find a body’s place and model the turning heavens.