The Cosmos in your Hand - A history of the astronomical teaching model

Kevin Johnson, Science Museum, South Kensington, on 2 November 2001

In the Ancient World celestial globes were constructed to represent the firmament. The first extant celestial globe is that on the shoulders a copy of a statue known as the Farenesi "Atlas", dating from 150 AD. Although, when we look at the sky, we see the heavens as if we were inside the celestial sphere, most celestial globes show the stars as if the observer were outside God's view of the heavens, as it were. This means that the star patterns or constellations are depicted laterally reversed, mirror images of the patterns seen by an Earthly observer. However, the globes used by the Arabs to teach astronomy after the fall of the Roman Empire were contrary to this as clearly shown by a bronze (the stars being inset metal studs) 17th century Persian globe. The earliest extant printed celestial globe dates from 1532. The Chinese also used large bronze celestial globes but these depicted many more and quite different constellations to those familiar to European astronomers.

The problem of longitude produced inaccurate maps and therefore terrestrial globes are much later than celestial ones. The Science Museum has a terrestrial globe made in Nuremberg in 1492. Such globes were very expensive at this time and therefore not used for education. It was not until the 18th century that terrestrial and celestial globes were made in large numbers. At that time many new constellations appeared on them the Microscope, the Air-pump, Herschel's Telescope etc. - some of which became redundant as a result of regulations agreed by international astronomical bodies in the 19th century.

George Adams made globes in pairs (terrestrial and celestial globes in matching mounts). A glass globe made for George III is still intact.

Although Sir Christopher Wren is depicted with a lunar globe in his portrait in the Sheldonian, Oxford, and the German, Schmidt, made one 19 feet in diameter with craters and mountains in relief, lunar globes fell out of use because the far side of the Moon was not known until the space programme revealed its complete surface in the latter part of the 20th century.

Only the top layer of the atmospheres of Jupiter and Venus are observable so globes are out of the question. Mars, however, does display permanent features and, in the 19th century, Schiaparelli described the straight lines he saw as `canali'. This word was mistranslated in America as canals, which excited everybody with the possibility of intelligent `Martians'. Schiaparelli named many Martian features but current nomenclature is due to Flammarion. For example, Schiaparelli's `Nix Olympia' we now know as `Olympus Mons'.

Armillary spheres explained the principles of an Earth-centred universe and were later modified to explain a universe centred on the Sun. The early Earth-centred ones were made of brass bands forming various `circles' by which the positions of celestial objects are determined. The circles include the celestial equator, circles of `declination' (analogous to terrestrial latitude) and great circles
representing `right ascension' (analogous to terrestrial longitude). In addition the great circle of the `ecliptic' (the path of the Sun in its apparent yearly journey round the Earth) was included.. The Leyden Sphere at the Museum of Voorhafen also depicts the Zodiac along the ecliptic.

The possession of globes or armillary spheres was considered to be a status symbol and Hans Holbein surrounds the subject of one of his most famous portraits with globes, mathematical devices, astronomical instruments and an armillary sphere (while also hinting at mortality by an oblique projection of a skull at the subject's feet!).

Following the Copernican revolution, these spheres became Sun-centred and included the paths of the inner planets - Mercury, Venus and the Earth and went on to explain the motions of the planets after Newton's ideas. These were later developed into the mechanical models known as orreries.

George Graham made a simple mechanical model showing the motions of the Earth and Moon round the Sun in the early 18th century for John Boyle, the Earl of Orrery - hence the name. Such devices became more and more elaborate, including the superior planets: Mars, Jupiter (with the Galilean satellites) and Saturn. Ramsden made orreries which displayed the gear-work and which reinforced the current view of the Universe as God's wonderful machine that followed Newton's `laws' of motion. In the Science Museum there is one made for George III and which was modified to include the planet Uranus (Georgium Sidus at the time). Orreries went out of fashion only in the 19th century. William Pearson used epicyclic gears in order to produce the planets' varying orbital speed resulting from elliptic motion.

These devices were important for lectures and demonstrations by peripatetic lecturers such as Demainbray who popularised `Natural Philosophy'. On entering the Science Museum one is confronted by a picture of an original painting by Joseph Wright of Derby of such a demonstration using an orrery. Eventually, cheaper orreries, armillary spheres and globes were made of wood with papered surfaces and began to be used in schools and colleges for the teaching of astronomy.

Orreries were supplemented by other mechanical models such as the `cometarium' which demonstrated Kepler's second law of planetary motion - most prominently displayed by comets - which states that the line between a planet and the Sun sweeps out equal areas in equal times. William Huggins' orrery was a terrestrial globe and the mechanism demonstrated the principle of precession. Bradley, the Third Astronomer Royal, made a model to demonstrate the aberration of light. A model which demonstrated the transit of Venus was made for the Royal Society to promote expeditions to observe the phenomenon (see the report of Martin Griffiths' October lecture.)

Today, such mechanical models still have their fascination. We admire and marvel at the invention and delicate workmanship with which they are contrived in the same way that we appreciate the beauty and ingenuity of watches and clocks - to which, of course, they are related. However, the mechanical watch has now been largely ousted by the quartz-driven chip (itself a marvel of invention and workmanship but with a beauty not so readily appreciated by the eye). Similarly, the orrery and other mechanical models have been superseded by, first the planetarium, but most recently buy the computer program.

Recent television presentations have dazzled us with computer simulations of aspects of the Universe but do we not recall with fondness the simpler mechanical models with which Patrick Moore illuminated us in the earlier episodes of "The Sky at Night"? Such models make the Universe tangible.

Richard Phillips