Humboldt State University ® Department of Chemistry

Richard A. Paselk

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.
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.
The Sphere
In the discussion below the numbers refer to the parts in the numbered view:
The Stand
The Stand serves three purposes: a) it is decorative, b) it serves as a memory aid to place the sun in the proper astrological house for a given date, and c) using the Meridian circle with the Horizon circle as reference, it allows one to set the inner sphere to a given latitude to show times of sunrise and sunset etc.

Instruments Medieval Science & Scientific Instruments


© R. Paselk
Last modified 9 January 2002