Armillary sphere

An armillary sphere is a model of the celestial sphere made from rings representing circles such as the celestial equator and ecliptic. It can illustrate the apparent motions and coordinates of celestial objects.

Celestial sphere: An imaginary sphere surrounding Earth onto which celestial objects are projected to describe their positions. The sphere supplies the geometric framework that the armillary’s rings model.
Meridian (astronomy): A great circle on the celestial sphere passing through the celestial poles and the observer’s zenith. A meridian ring can establish the sphere’s orientation relative to an observer.
Ancient Greek astronomy: The study of celestial phenomena by Greek thinkers and observers from antiquity through late antiquity. Greek astronomy developed geometric models that informed early armillary instruments.
Astrolabe: An instrument that models the celestial sphere on a flat, rotating plate for solving astronomical problems. Unlike an armillary sphere’s three-dimensional rings, an astrolabe compresses celestial geometry into a plane.
Portuguese heraldry: The system of coats of arms and heraldic symbols associated with Portugal and its institutions. The armillary sphere became a prominent emblem in Portuguese royal and national heraldry.
Celestial equator: The projection of Earth’s equator onto the celestial sphere. A prominent ring marks this reference circle on many armillary spheres.
Celestial poles: The two points where Earth’s rotational axis, extended, intersects the celestial sphere. The polar axis supports the rotation represented by many armillary models.
Hipparchus: A Greek astronomer of the second century BCE known for systematic star cataloguing and mathematical astronomy. Ancient accounts associate Hipparchus with the construction and use of celestial instruments.
Orrery: A mechanical model showing the relative motions and positions of planets in the Solar System. An orrery models planetary motion, while an armillary sphere emphasizes celestial circles and coordinates.
Portuguese discoveries: The Portuguese maritime expansion and exploration that connected Europe, Africa, Asia, and the Americas. The sphere’s association with navigation and empire grew during Portugal’s overseas expansion.

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