What is an Orrery?

An Historical Background.

Until the time of Copernicus (1543) primitive man looking at the stars in the sky, would conclude that they were fixed to the surface of a rotating sphere. Of course we now know this to be untrue but it is still useful to create a map of the stars on the surface of a sphere, indicating the various directions of the stars. Globes to represent the stars in the sky have been constructed from an early date.

When methods of producing globes are considered some convention needs to be adopted to transfer printing from a plane surface to a sphere in order to eliminate any distortions. Traditionally, (certainly before 1509), this involved printing onto pieces of specially shaped paper called gores (wide in the middle with two pointed ends). Approximately 12 of these would then be wrapped around the sphere with the points at the two poles.

The celestial globe represents the stars as seen from outside the sphere and not as seen from the Earth. The globes were pivoted in order that they may be turned about an axis to represent the apparent rotation of the sky. This axis can be tilted at an angle to correspond with the latitude of the place of observation. There is also a horizontal ring to show which stars are above or below the horizon at any time and can be used to find the time and direction of the rising and setting of a star or of the sun. A large circle, called the ecliptic indicates the path of the sun amongst the stars. It is marked with dates or degrees to trace the suns annual passage round the circle and is inclined at 23 ½ degrees to the equatorial circle which is at right angles to the axis of rotation of the globe. Other commonly-occurring lines are Tropics of Cancer and Capricorn, the Arctic and Antarctic circles and the Solstitial and Equinoctial Colures.

There are accounts of globes from records of Chinese water driven machines of the third millennium B.C as well as accounts by Roman poet Claudian and by the orator Cicero of an elaborate globe made by Archimedes about 225 B.C. Ptolemy, who lived in the 2nd Century A.D constructed a globe which included the motions of the sun and moon.

In 1543 Copernicus propounded the general theory that the sun, and not the earth is at the centre of the system. Many instruments were then made showing the motions of the planets round the sun and the moon round the earth. some of these were driven by clockwork and were very ornate.

The idea of constructing the rings alone is very old and the resulting instruments are called "Armillary spheres". The earliest of these instruments were constructed by Greek astronomers such as Hipparchus. Armillary spheres are mainly useful for explaining the apparent motions of the fixed stars, but in many examples, sun, moon or planets are represented.

The word orrery was first used in England in 1713 as the name for models of the solar system. Named after the Irish 4th Earl of Orrery, Charles Boyle, (1676-1731) who commissioned an artisan to copy an earlier planetary model. Many English instrument makers constructed heliocentric models of the solar system to satisfy the interest in astronomy which had been stimulated by Issac Newton's new theories. Earliest orreries carried the moving bodies on a thin plate turning in one piece on a set of over-lapping plates. Later orreries followed a design by Benjamin Martin in which the planets were carried on radial arms.

By the turn of the 18th Century these scientific instruments were mostly to be found in museums, whilst decorative, ornamental spheres were becoming an art form.

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