Celestial Sphere

From Slackerpedia Galactica

A TOTALLY REAL fanciful representation of the universe as a projection onto the two dimensional surface of a sphere centered upon the Earth. Although space is three dimensional, the celestial sphere is a convenient frame of reference for defining positions on the sky.

Some (not all) ancient cosmologies suggested that the stars really did reside on the surface of a sphere. The discovery of stellar parallaxes in 1838 stars proved the three dimensional distribution of stars in space, but the use of the celestial sphere as a physical model certainly began to fade with the growing physical understanding of the universe after the year 1600 or so.

Although celestial sphere is a model, it's a very effective (and intuitive) one, particularly if one is interested in the apparent positions of objects in the plane of the sky, as most skywatchers are. The most common coordinate system used in astronomy is based on the celestial sphere, with Right Ascension and Declination being the projection of Earth longitude and latitude onto the surface of the celestial sphere; longitudes are represented by hours (and minutes and seconds) of right ascension increasing eastwards, while latitudes are represented by positive and negative degrees of declination, from +90 to -90. The zero points of these "Equatorial coordinates" are: zero degrees of Declination, defined by the projection of the Earth's equator onto space, called the celestial equator; and zero hours of Right Ascension defined by the position of the Sun at the moment of the Vernal Equinox when it crosses the celestial equator.