Ancient Chinese sundial.
Eratosthenes is credited as the inventor of the armillary sundial. He used it as a teaching device and its use was continued by such great early astronomers a s Hipparcos and Ptolemy. In its simplest form, consisting of a ring fixed in the plane of the equator, the armillary sundial is one of the most ancient of astronomical instruments. Slightly developed, it was crossed by another ring fixed in the plane of the meridian. The first was an equinoctial, the second a solstitial armilla. Shadows were used to indicate the sun’s positions, in combinations with angular divisions. When several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere.
The armillary sundial continued to evolve, creating progressively more complicated armillary spheres throughout medieval and Renaissance times. For many years they represented the height of wisdom and knowledge. The armillary sphere survives as a useful teaching tool. It can be described as a skeleton celestial globe, the series of rings representing the great circles of the heavens, and revolving on an axis within a horizon. With the earth as center such a sphere is known as Ptolemaic; with the sun as center, as Copernican.
Although the armillary sundial has been relegated to the status of a garden decoration, it helped to revolutionize astronomy in the early centuries before Christ. Here on Universe Today we have a great article about a related tool: the astrolabe. Astronomy Cast offers a good episode about something that would have puzzled ancient astronomers: the large scale structure of the universe.
Sources:
http://www.mlahanas.de/Greeks/Eratosthenes.htm
http://en.wikipedia.org/wiki/Sundial
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