This replica combines two armillary spheres into one, containing models for both the Sun and the Moon. The annual motion of the Sun along the “ecliptic,” its apparent path around the sky, is inclined to the Earth’s equator by 23.5°. The motion of the Moon follows a path inclined by 5° to the plane of the ecliptic. This instrument also demonstrates the precession of the equinoxes.
Armillary spheres, serving diverse purposes, were made in many sizes and designs, with different numbers of rings and various accessories. For ancient astronomers, a simple armillary sphere represented the fundamental circles of the sky, including the local horizon, the celestial equator, the tropics of Cancer and Capricorn, and the ecliptic (or apparent path of the Sun). Using such an instrument, with the Earth at the center, one may observe the positions of stars, demonstrate the motion of the Sun, and calculate the position of the Sun, bright stars and Zodiac constellations for any date.
By the 16th century, the crafting of ever more complex armillary spheres in brass and wood reflected attempts to realize, in material form, a functioning model of the celestial orbs described in astronomical works such as Peurbach’s Theorica nova planetarum. Such complex instruments blur the boundaries between armillary spheres and orreries. One may be referred to as an “orbarium” or “planetolabium.”
This replica is based on an instrument created in Amsterdam between 1725-1750 by Leonhard Gerhard Valk and now held in The National Maritime Museum Collection at Greenwich, England (ASTO625).
- Horizon system
At the top of this 2-ft tall instrument, four arms support a horizon ring (60 cm diameter). Diverse scales inscribed on the horizon ring indicate degrees, zodiacal sign, names of the month (Latin), and names of the 12 winds (Latin and Greek). The horizon ring intersects a vertical meridian ring, numbered in 10° increments north or south of the horizon.
2. Primary sphere: Equatorial system
Within the horizon ring lies the primary sphere (24.5 cm diameter), comprised of 8 brass rings. One of these rings, the celestial equator, is divided into 360°. Four other rings lie parallel to the celestial equator: the polar circles and the two tropics.
Two rings of the primary sphere are fixed perpendicular to the celestial equator, including the equinoctial colure, which is marked with a scale of declination (degrees north or south of the celestial equator). Declination is numbered at every 10° increment from 0° to 90°. Declination may be measured down to 1° divisions.
The primary sphere also features a zodiacal band attached to the colure rings. It is divided into twelve regions, each 30° long, bearing the Latin names and figures of the Zodiac constellations.
3. Ecliptic system
Within the primary sphere, another movable system of rings is based on the poles of the ecliptic, or path of the Sun. This system is angled to the equatorial system of the primary sphere, where two short axes indicate the offset north and south ecliptic poles. The ecliptic system consists of 4 rings which model the motions of the Sun and the Moon.
One ring, not inscribed, lies in the plane of the ecliptic, to model the motion of the Sun. Two rings parallel to the ecliptic ring represent circles of latitude, at a given distance north and south of the ecliptic.
The 4th ring is angled to the ecliptic to model the motion of the Moon. This Moon ring is inscribed: “Nodus (dragon head) euehens” and “aux.” An opposite inscription reads: “Nodus (dragon tail) deuehens” and “Oppositum augis.”
In the center, the Earth appears as a globe, with geographical detail (GLB0246). The globe is inclined 23.5° to offset (or cancel) the ecliptic system. As a result, the poles of the Earth always point to the equatorial poles of the primary sphere. William Dampier’s explorations north of New Guinea are shown as Britannia Nova. California appears as an island.
On the base of the instrument, a compass lies at the center of a 32-point Dutch wind rose. The outer edge contains the names of winds in two languages.