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Old 09-04-2012, 01:33 AM   #92
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Lightbulb Celestial sphere

In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the observer. All objects in the observer's sky can be thought of as projected upon the inside surface of the celestial sphere, as if it were the underside of a dome or a hemispherical screen. The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant.

Imagination, also called the faculty of imagining, is the ability of forming images and sensations when they are not perceived through sight, hearing, or other senses. Imagination helps provide meaning to experience and understanding to knowledge; it is a fundamental faculty through which people make sense of the world, and it also plays a key role in the learning process...A sphere (from Greek σφαῖρα — sphaira, "globe, ball") is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle, which is in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...This distance r is known as the "radius" of the sphere.It passes through the center and is thus twice the radius.

These concepts are important for understanding celestial reference systems, the methods in which the positions of objects in the sky are measured. Certain reference lines and planes, as projected onto the celestial sphere, form the basis of the reference systems. These include the Earth's equator and axis, and the Earth's orbit.

Basics of Astronomy: The Celestial Sphere ...The date recorded as 72 after the Hijra (or 691–692 CE), which historians view as the year of the Dome's construction...

Because astronomical objects are at such remote distances, casual observation of the sky offers no information on the actual distances. All objects seem equally far away, as if fixed to the inside of a sphere of large but unknown radius, which rotates from east to west overhead while underfoot, the Earth seems to stand still. For purposes of spherical astronomy, which is concerned only with the directions to objects, it makes no difference whether this is actually the case, or if it is the Earth which rotates while the celestial sphere stands still.

Earth's motion around the Sun...

The ancients assumed the literal truth of stars attached to a celestial sphere, revolving about the Earth in one day, and a fixed Earth. The Eudoxan planetary model, on which the Aristotelian and Ptolemaic models were based, was the first geometric explanation for the "wandering" of the classical planets. The outer most of these "crystal spheres" was thought to carry the fixed stars. Eudoxus used 27 concentric spherical solids to answer Plato's challenge: "By the assumption of what uniform and orderly motions can the apparent motions of the planets be accounted for?

In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years (called a Great or Platonic Year in astrology). The term "precession" typically refers only to this largest secular motion; other changes in the alignment of Earth's axis — nutation and polar motion — are much smaller in magnitude.

Last edited by lightgiver; 09-04-2012 at 01:44 AM.
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