An armillary sphere is a representation of the celestial sphere that was used to describe the motions of the stars and planets across the sky. The word armillary is derived from the Latin armillae meaning a bracelet. The sphere is made up of several concentric rings (or bracelets) set inside one another which collectively represent an Earth centred view of the celestial sphere.
Armillary spheres were developed by the Greeks and were used as teaching aids to help visualise the movement of stars and planets in the heavens as early as the 3rd century BCE. Larger and more precise machines were also used as observational instruments. The Chinese had also developed simple devices as early as 4th century BCE. These developed in style and complexity over the ensuing centuries culminating in the construction of the Honcheonsigye armillary in 1433. This is the only astronomical clock from the Joseon Dynasty in existence today.
The Persians and Arabs improved the device even more and developed other astronomical instruments such as the astrolabe. More refinements occurred during the Renaissance particularly by Tycho Brahe (1546-1601).
A description of the garden armillary and more photos can be found on this page.
Garden armillary sundials
Another form of garden armillary is a sundial. Usually it is simply two open rings set at right angles to each other with a rod (or gnomon) running down the central axis which casts a moving shadow along the horizontal ring section as the Sun traverses the sky. It is important to align the gnomon with the southern and northern celestial poles so that its axis will be parallel with the north/south axis of the Earth.
Below are some photos taken during construction showing the ring orientation.