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.
