## 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:
Materials:
• The sphere itself is defined by three rings: the the Equinoctial Colure (9), the Solstitial Colure (10), and the Equator (5). These rings were fabricated from heavy (8 G; 1/8") brass sheet cut into 1/4' wide strips.
• The broad band of the Ecliptic (6) was fabricated from a thin (20 G) brass sheet cut to give a 3/4" wide strip.
• The Arctic Circle (3), Tropic of Cancer (4), Tropic of Capricorn (7), and Antarctic Circle (8) were made from brazing rod (11 G; 0.91" dia).
• The axis (1) was made from a piece of 1/8" brass rod.
• The Meridian circle (2) was cut from a solid brass 3/8" thick plate using a machinists lathe to give a 0.20" thick x 3/8" deep ring.
• The central ball (the "Earth") is a 1 1/2" Malachite sphere purchased at a "nature" store.

Construction:
• To make the 4 3/4" o.d. sphere three strips of the 1/4" 8G brass were cut to identical length. I calculated the desired length for the circumference by multiplying the diameter (4 5/8": center-to-center of strip) by pi and then added about 1/2" to accommodate a 1/4" lap joint. One-quarter inch steps were then filed half-way through on opposite sides of the two ends of each piece. The pieces were then bent into circles on a wood arbor (I made mine with a hole saw out of 3/4" wood plank), then riveted together using 1/8" brass rod for the rivets. I drill the holes the same diameter as the rod for a tight fit, the counter-sink them slightly on the exposed sides, the rivets will then mushroom over for a permanent joint. The two colure rings were then notched on opposite sides at the north and south poles to allow them to interlock at right angles. Both colures were then notched on the outside at the equator, while the equator ring was notched on the inside at four places corresponding to the two colures. The equator ring was then snapped in place.
• A length (pi x 4 3/4") of 3/4" 20G brass was cut and then laid out into 12 segments corresponding to the 12 houses of the zodiac. These segments were then marked off and graduated at 5° intervals with a scribe. Each segment was then labeled with a stamp set. The ecliptic band was then shaped on an arbor and butt-silver soldered.
• Two 2" diameter and two 4 3/8" diameter circles were then made by cutting the appropriate lengths of brazing rod and butt-silver soldering them after forming on arbors. Notches were then cut with a file at the appropriate angles on the two colures (eight notches on each), and the new rings were fitted to them and soldered in place (use a lower temp silver solder, or tin solder). The ecliptic band was now attached by riveting it where the equinoctial colure and equator cross (Autumnal and Spring equinoxes). The ecliptic can then be adjusted so that its center aligns with the rings for the tropics.
• For the axis a 5" length of 1/8" brass rod was cut in half. Each half was then bored on one end to hold a 1/16" piece of rod which would insert into the central malachite "earth." The opposite end of each piece was decorated with a ring and spherical end by holding in a chuck and shaping them with a small file by hand.
• The meridian ring was divided to single degrees, with major divisions every five degrees and numbered every ten degrees. Holes 1/8" diameter were drilled at the top (90° N) and bottom (90° S) to accomodate (friction fit) the axis rod.
• The central ball is a 1 1/4" diameter malachite sphere. A 1/16" hole was bored through its center with a standard carbon-steel twist drill. This requires patience and some luck, as malachite, though soft, is also brittle and can easily chip or shatter. For best results drill at a slow speed and remove the dust frequently. Malachite is a copper salt (copper II carbonate hydroxide), so avoid breathing the toxic dust. A little water or oil can help keep the dust under control as a sludge or paste.

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.
Materials:
• The large ring representing the Horizon, was fabricated from 1/4' thick cast bronze plate. (Recycled from a discarded grave marker!)
• The supporting arms were made from two 12" lengths of 1/4' square "nickel silver" rod recovered from old rheostats.
• The center brass pillar is of two pieces, a 1 3/8" dial lamp piece and a smaller piece fabricated from 1/2" brass round stock.
• The wood base was turned from two pieces of oak and glued together.

Construction:
• A rough circle was first cut from the bronze plate with a hand "jig" saw. The circle was then mounted on a lathe and turned to 8" diameter with a lightly chamfered edge. The plate was then surfaced on what would be the top side from about 2 1/2" from the center to the outside. Starting from the center, six bands were now laid out and scribed (see illustration above): first, at about 2 3/4" from the center, is a 3/16" band for the days of each month; the next band, again about 3/16" is for the names of the months; third, is a wide band, about 3/8" for the signs of the zodiac; next comes another narrow (3/16") band for the degrees in each sign, graduated from 0 - 30 by five and numbered; then is a very narrow (1/16") band graduated into 360°, and finally there is a 3/16" band graduated every five degrees and numbered from 5 - 355, with eight points of the compass marked with letters (N, NW, etc.). Notches were cut with a file at two locations 180° apart to accommodate the meridian ring.
• The two supporting arm arcs were bent from the center over an arbor to give seven inch diameter arcs with approximately one inch straight extensions on each end. Short lengths of 1/8" German silver rod were affixed in the holes already present in each end to serve as rivets. The two arcs were hen notched oppositely to cross-over each other and a hole was drilled through the junction to attach them to the stand.
• The brass cap on the center at the stand was turned from a short bit of 1/2" diameter brass rod. It was shaped by hand with round and flat files. The top was then notched with a file to fit the meridian ring, and the bottom was drilled and tapped to fit the brass rod holding it to the oak stand.
• The two pieces of the stand, a 5" diameter base, and 1 1/2" diameter pillar. were turned separately on a wood lathe and glued together. A hole was drilled up through the center of the assembly and countersunk to accommodate a threaded brass rod and nut to hold the metal assembly in place. Grooves were dadoed at 90° to fit the arcs of the stand.

 Instruments Medieval Science & Scientific Instruments References