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.
How it works
The astrolabe is a flat map of the celestial sphere, projected onto a disc. A pierced, rotating plate called the rete carries the brightest stars and the ring of the ecliptic; turning it reproduces the sky’s daily rotation. A sighting bar, the alidade, measures the altitude of a star or the Sun.
What it was used for
With it you could tell the time by day or night, find the rising sign and the astrological houses for a moment, survey, and take bearings. For centuries it was the essential instrument of both astronomers and astrologers.
Its history
The projection behind it goes back to Hellenistic Greece and to Ptolemy; the instrument was then developed and refined extensively in the medieval Islamic world before spreading through Europe.
Connections
- 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.
- Claudius Ptolemy — A Greco-Roman mathematician, astronomer and astrologer of the second century, working in Alexandria. His Almagest set out the geometry of the heavens that Europe and the Islamic world used for over a thousand years, and his Tetrabiblos became a foundational text of Western astrology.