The instrument collection of Charles Boyle, fourth Earl of Orrery (1676-1731), could keep this column supplied with spheres of different kinds for many years to come. Bequeathed to Christ Church and placed on loan to the Museum by the College, it is one of the most important surviving examples of a cabinet of instruments, coming from a period when one manifestation of a fashionable interest in natural philosophy was the assembling of just such collections by wealthy amateurs.
John Rowley (d. 1728) was one of the leading London instrument makers of his time and is prominently represented in the Orrery Collection. The example of his work chosen here is a silver armillary sphere, that is, a skeletal sphere made up of rings (armillae) that represent the prominent circles of the celestial sphere. These include the equator, the tropics and the ecliptic (the apparent annual path of the sun through the stars). This spherical assembly can rotate on an axis supported within a vertical ‘meridian ring’, which is itself held in a stand with a flat ring that represents the horizon.
The meridian ring can be adjusted, so as to set the elevation of the pole – the point about which the sphere can rotate – for any latitude. Once this is done, the rotation of the sphere represents, for example, the rising of the stars and the sun above the horizon, their culmination on the meridian, and their setting.
On the instrument by Rowley, the meridian ring is graduated, as is normal, in degrees above and below the equator (i.e. for declination). In addition, however, a spherical triangle made of brass strips is provided, which rests loosely on the horizon ring and rises to the zenith, with graduations for altitude above the horizon.
Rowley’s instrument also incorporates a more noteworthy and substantial addition than the novel feature of the spherical triangle. Within the customary representation of the celestial sphere, Rowley provides a model of the planetary system or ‘orrery’ – the term used to describe a moving planetary model that was in fact derived from another instrument made by Rowley for the fourth Earl.
The standard armillary sphere provides a Ptolemaic model of the cosmos, with the heavens rotating around a central, stationary earth. In the early eighteenth century, however, attempts were made to combine the arrangement of armillae with the Copernican system of the planets, where the earth revolves around the central, stationary sun.
In his silver armillary sphere and orrery, Rowley presents an ambiguous solution to the problem of representing the Copernican system in a form that will be recognized as a model of the heavens. He places the sun at the centre, while Mercury, Venus, the earth and moon, Mars, Jupiter, and Saturn are arranged so as to revolve in the plane of the ecliptic. Symbols for the inferior planets and a globe for the earth are supported on an axis extending from the southern ecliptic pole and symbols for the superior planets are carried on skeletal hemispheres pivoted at the north; they all therefore orbit in the plane of the ecliptic, which must now represent not the annual path of the sun through the stars, but the projection of the plane of the earth’s orbit onto the celestial sphere. Yet in this model, the celestial sphere still rotates about the celestial poles, as it can do only in the old Ptolemaic arrangement.
Is Rowley’s instrument an uncomfortable amalgam of two incompatible ways of considering the cosmos, or is it rather a subtle and clever didactic tool, which can introduce and compare the systems of Ptolemy and Copernicus in a single model?
Since the Orrery Collection has a similar sphere by Rowley – larger and made of brass – which is paired with a Ptolemaic sphere, it could be argued that the silver armillary sphere and orrery design is meant to be Copernican, rather than dual-purpose. While a number of functions, such as setting the celestial sphere for a particular time, require the rotating armillae, the price for allowing such features is an inconsistent model of the heavens.
The same problem is present in other attempts to design Copernican armillary spheres in the period. Such instruments are rare and were soon displaced by the orrery proper, where, if the celestial sphere is represented by rings or arcs at all, they are fixed.
Rowley’s armillary sphere and orrery is predominantly made of silver, with brass symbols for the planets and the moon. The earth (a sphere of ivory) is linked to the sun (a sphere of brass) by a small train of gear wheels so that it rotates on its axis as it is moved in its orbit. This points again to the instrument’s ambiguities: the celestial sphere has an axis (Ptolemaic), but so does the earth (Copernican).
The stand is of silver and ebony and has an inset magnetic compass in its base for orienting the meridian ring. A shaped case of lignum vitae has ivory and brass fittings. The diameter of the horizon ring is 123 mm and the instrument is signed on the meridian ring ‘I. Rowley Fecit’.This Copernican armillary sphere may present conceptual problems, but it is a beautiful and finely executed instrument. The Museum’s first Curator, Robert Gunther, considered it “probably one of the finest small astronomical models ever made in England”. [J. A. B.]