An armillary sphere (also known as spherical astrolabe) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latinarmilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth.
Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments, being preferred by Ptolemy. Armillary spheres became popular again in the late middle ages; the Danish astronomer Tycho Brahe (1546-1601) constructed several of such instruments.
Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the height of wisdom and knowledge.
Chinese Armillary sphere
Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices.
A representation of an armillary sphere is present in the modern Portuguese flag and has been a national symbol since the reign of Manuel I.
The celestial sphere is divided by the celestial equator.
In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxial with the Earth.
In the latter case it is centred around an observer on the surface of the Earth and then horizontal parallax cannot always be ignored; especially not for the Moon.
Hipparchus was the first to show that the stereographic projection is conformal, and that it transforms circles on the sphere that do not pass through the center of projection to circles on the plane.
Hipparchus seems to have used a mix of ecliptic coordinates and equatorial coordinates: in his commentary on Eudoxos he provides the polar distance (equivalent to the declination in the equatorial system) and the ecliptic longitude.
He confirmed that precession affected the entire sphere of fixed stars (Hipparchus had speculated that only the stars near the zodiac were affected), and concluded that 1° in 100 years was the correct rate of precession.
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