Celestial Sphere Facts

Celestial Sphere Facts
A celestial sphere is an imaginary sphere used for spherical astronomy. It has a gigantic radius and the earth is its center, with poles aligned with the earth's poles, and its equator lies on the same plane as the equator of the earth. The celestial sphere is essentially a large sphere that surrounds the earth allowing us to maintain references to celestial bodies located in the sky. Any object in the celestial sphere can be located by having two coordinates including the Declination and Right Ascension. It is easiest to think of the celestial sphere as the underside of a dome.
Interesting Celestial Sphere Facts:
Ancient humans believed that stars were attached to the celestial sphere and that they revolved around the planet in only one day. They also believed that the earth was fixed.
The first model that explained how classical planets wandered was the Eudoxan planetary model. The classical planets include the Moon, Mercury, Saturn, Jupiter, Mars, Venus, and the Sun.
Aristotle and Ptolemy planetary models were based on the Eudoxan planetary model, with the fixed stars carried by the outer crystal spheres.
Some also refer to the celestial sphere as a celestial globe. These globes map the outside constellations, showing a mirror image of what they would look like from earth.
The oldest remaining celestial globe dates back to a 2nd century sculpture called the Farnese Atlas - given the name because it was acquired by Cardinal Alessandro Farnese in the 1500s.
The celestial sphere has an equator and poles, just like the earth.
The North Celestial Pole (NCP) is the same as the earth's North Pole only projected much further into space. The South Celestial Pole is the same as the earth's South Pole only projected much further into space as well.
The Celestial equator is the same as the earth's equator but it extends much further along the same plane out into space.
The zenith of the celestial sphere is the point directly overhead on the celestial sphere.
The meridian of the celestial sphere is the line extending from the horizon's north point, extending up through the zenith and then down to the horizon's south point.
The celestial coordinates are the Declination and the Right Ascension. Any object on the celestial sphere can be located with these two coordinates.
The Declination of a celestial sphere object is measured either south or north from the equator's plane. Therefore the equator's declination is 0 degrees; the South Celestial Pol is -90 degrees; the North Celestial Pole is +90 degrees. This is parallel to earth's latitude.
The Right Ascension of a celestial sphere object is measured along the celestial equator, with the zero point being the vernal equinox or where the sun is on the first day of spring. This measurement is always in hours, minutes, and seconds. One hour equals 15 degrees because there are 24 hours in a day and the celestial sphere rotates 360 degrees in one day.
Galaxies and stars are almost fixed in their Right Ascension and Declination positions, while planets and suns have coordinates that change throughout the year.

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