Replicas and Restorations of Early Instruments by Richard A. Paselk

Richard Paselk, Curator

The Armillary Sphere

An armillary sphere is basically a skeletal celestial sphere with a model of the Earth or, later, of the Sun placed in the center. It is useful as a teaching tool and as an analog computer for solving various astronomical problems to a crude degree of accuracy. Armillary spheres were developed by the Greeks in antiquity for use as teaching tools. In larger and more precise forms they were also used as observational instruments, being preferred by Ptolemy. Armillary spheres became popular again in the late middle ages. With the advent of the Copernican model of a Sun centered Universe pairs of spheres contrasting the Copernican and Ptolomaic models became common teaching/demonstration tools. Such small teaching sphere remained popular through the nineteenth century. An excellent discussion of celestial and armillary spheres is found in Chapter 2 of Evans*.

Description

The Armillary sphere consists of two major components, the sphere and the stand, as seen in the figure above. The heart of the Armillary sphere is the sphere itself, which was often made and used alone. Renaissance painters frequently show spheres on handles in paintings of scholars etc. The central body in the sphere represents the Earth, which was, of course, considered the center of the Universe. The colures and the Equator (the rings defining the sphere) represent the firmament, that is, the sphere upon which the fixed stars reside. The band going around the sphere, at an angle to the equator, represents the zodiac. The line running through the middle of this band defines the ecliptic, or the path followed by the Sun through the sky. The width of the band is ideally about ±9° to include the wandering of the Moon and planets above and below the Sun's path. The various constellations of the Zodiac also fall along this band.

*Evans, James. The History & Practice of Ancient Astronomy. Oxford University Press, Oxford (1998). A wonderful new book which looks at ancient (particularly Greek, but spanning Babylonian to Medieval Europe) astronomy in context. Not only does the author provide many translations of how astronomy was actually done by the ancients, he also explains how to make observations of the celestial sphere using ancient tools and home-made modern counterparts. Tools explained range from the very simplest (the gnomon) to the complex (armillary spheres, astrolabes, planetary equatoria). An appendix even provides diagrams for photocopying to make an astrolabe and an equatorium for Mars.

© R. Paselk 2013, Last modified 26 December 2020