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Date Posted: 15:55:27 12/29/07 Sat
Author: DEAN WHITE
Subject: Re: What is an orrery?
In reply to: Dean WHite 's message, "What is an orrery?" on 09:36:17 06/28/02 Fri

>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 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
>elliminate 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 whcih is at right angles to the axis
>of rotation of the globe. Other commonly-occuring
>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.
>Earlisest 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.
>
>To find out more about orreries go to my homepage
>www.planetaria.co.uk

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