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.
Medieval Science & Scientific
Instruments