Celestial coordinate system Reference Libraries

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Prime Meridian
2013-02-19 15:54:46

A prime meridian is a meridian, i.e. a line of longitude, at which the definition of longitude is zero degrees. A prime meridian and its opposite in a 360 degree-system, the 180th meridian, create a great circle. This great circle separates the sphere into two hemispheres. If one uses directions f East and West from a defined prime meridian, then they can be named Eastern and Western...

2013-02-19 15:46:08

An equator is the intersection of a sphere’s surface with the plane that is perpendicular to the sphere’s axis of rotation and midway between the poles. The Equator is normally referred to the Earth’s equator: an imaginary line on the Earth’s surface halfway between the North Pole and the South Pole, separating the Earth into the Northern Hemisphere and Southern Hemisphere. Other...

2014-01-12 00:00:00

Sample Entry: Astronomy is the scientific study of stars, planets, comets, galaxies, and other phenomena that occur outside Earth's atmosphere (e.g. cosmic radiation). Astronomy deals with the evolution, physics, chemical makeup, meteorology, and motion of celestial objects, and also the formation of the universe. The word Astronomy comes from the Greek words astron (meaning "star") and...

Autumnal Equinox
2004-10-19 04:45:44

Autumnal Equinox -- In astronomy, is the equinox at the beginning of autumn in the northern hemisphere: the moment when the sun appears to cross the celestial equator, heading southward. The equinox occurs around September 22-24, varying slightly each year according to the 400 year cycle of leap years in the Gregorian Calendar. In the southern hemisphere, the equinox occurs at the same...

2004-10-19 04:45:44

Zodiac -- The Zodiac (from Greek zoon, "animal") is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path, and including the apparent paths of the Moon and the major planets: Mercury, Venus, Mars, Jupiter, and Saturn. Imaginary divisions of the zodiac represent, in astronomy, constellations, and in astrology, signs. There is a zodiac in...

Ptolemaic System
2004-10-19 04:45:43

Ptolemaic System -- The Ptolemaic system was a system to explain the motions of the heavens, espoused by Claudius Ptolemaeus in Almagest sometime around the second century, C.E., and accepted for over a thousand years by the vast majority of people to be the correct cosmological model. Unlike earlier systems (such as 'the stars move because that is the will of the gods', or the model of...

2004-10-19 04:45:42

Precession -- Precession is the phenomenon by which the axis of a spinning object "wobbles" when a torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession. As a spinning object precesses, the tilt of its axis goes around in a circle in the opposite direction that the object is spinning. If the speed of the rotation...

Geocentric Model
2004-10-19 04:45:41

Geocentric Model -- The geocentric model of the cosmos is a paradigm which places the Earth at the center of the universe. Common in ancient Greece, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and planets orbit Earth. Similar ideas were held in ancient China. The geocentric model was gradually replaced by the heliocentric model of Copernicus...

2004-10-19 04:45:41

Ecliptic -- The ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is the great circle on the celestial sphere that contains the different positions of the Sun relative to the background stars throughout the year, and most planets can be seen close this circle. The zodiac...

Circumpolar Star
2004-10-19 04:45:41

Circumpolar Star -- Circumpolar stars are those stars which are located near the poles of the celestial sphere. As the Earth rotates, the sky appears to rotate; and most stars will be hidden below the horizon at some point in their circular paths. A circumpolar star is near enough to the celestial pole that it will never go under the horizon. It will therefore be visibile for all the night,...

Word of the Day
  • Any of various tropical Old World birds of the family Indicatoridae, some species of which lead people or animals to the nests of wild honeybees. The birds eat the wax and larvae that remain after the nest has been destroyed for its honey.
Honeyguide birds have even been known to eat candles.