The armillary sphere

Armillary spheres were invented to show the apparent movement of the Sun, the Moon and the planets around the Earth. An armillary is essentially a cut-open celestial globe with circles to represent the orbits of the celestial bodies. Many armillaries include circles for the horizon, meridian, Equator, the tropics, the polar circles and the ecliptic. The circules usually carried scales calibrated in degrees for measurement of the planets' positions.

Armillaries were designed by early astronomers. There is some debate about who designed the first armillary; most likely they were invented independently in Greece and in China at about the same time. According to the Encyclopaedia Britannica (1994) "the earliest known complete armillary sphere with nine circles is believed to have been the meteoroskopion of the Alexandrine Greeks (c. AD 140), but earlier and simpler types of ring instruments were also in general use. Ptolemy, in the Almagest, enumerates at least three. It is stated that Hipparchus (146-127 BC) used a sphere of four rings; and in Ptolemy's instrument, the astrolabon, there were diametrically disposed tubes upon the graduated circles, the instrument being kept vertical by a plumb line. "

The Chinese astronomer Chang Heng, who lived from 78 to 139, was the first to connect an armillary sphere to a water drive to make it rotate automatically at the correct speed, which allowed observers to read off the position of the planets at any time. In 1088 Su Song included an armillary in his water driven clock tower.

Armillaries became popular in Europe during the 17th and 18th centuries to demonstrate the difference between the Ptolemaic and the Copernican celestial systems. They evolved from devices developed by Arab astronomers during the Middle Ages and soon became more an object for showing off the open-mindedness towards new ideas of its wealthy owner than a scientific instrument. The illustration below shows such a display armillary.


China Internet Information Center (2001) Formation of the Chinese civilization: Astronomy and Mathematics. (read 23 November 2003).

Armillary sphere, Encyclopaedia Britannica (1994), 15th edition.