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In Greek antiquity the ideas of celestial spheres and rings first appeared in the cosmology of Anaximander in the early 6th century BC. [7] In his cosmology both the Sun and Moon are circular open vents in tubular rings of fire enclosed in tubes of condensed air; these rings constitute the rims of rotating chariot-like wheels pivoting on the Earth at their centre.
The spheres thus moved with a "natural nonviolent motion". [33] The mover's power diminished with increasing distance from the periphery so that the lower spheres lagged behind in their daily motion around the Earth; this power reached even as far as the sphere of water, producing the tides. [34] [35]
Visualization of a celestial sphere. In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth.All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer.
The second sphere explains the movement of the Sun and the Moon on the ecliptic plane. The third sphere was supposed to move on a “latitudinally inclined” circle and explain the latitudinal motion of the Sun and the Moon in the cosmos. Four spheres were assigned to Mercury, Venus, Mars, Jupiter, and Saturn, the only known planets at that time.
Astronomers believed that the seven naked-eye planets (including the Moon and the Sun) were carried around the spherical Earth on invisible orbs, while an eighth sphere contained the fixed stars. Motion was provided to the whole system by the Primum Mobile, itself set within the Empyrean, and the fastest moving of all the spheres. [5]
Many of the subsystems are characterized as "spheres", coinciding with the shape of the planet. The four spheres (for which most of the other spheres are a subtype of) are the atmosphere, the biosphere, the hydrosphere and the geosphere. Earth's ecosphere lies it self within the heliosphere (the Sun's astrosphere). [4]
In Aristotelian physics and Greek astronomy, the sublunary sphere is the region of the geocentric cosmos below the Moon, consisting of the four classical elements: earth, water, air, and fire. [1] [2] The sublunary sphere was the realm of changing nature.
This imposed a minimum radius for the sphere of fixed stars at center-to-center Earth to Moon distance plus the Moon's radius (approx. 1/3 Earth radius), plus the width of the Sun (it being, at least, the same that the Moon), plus the indeterminate thickness of the planets' spheres (believed to be thin, anyway), for a total about 386,400 km ...