Celestial sphere

Celestial Coordinate System

The celestial sphere is a theoretical sphere of huge radius, onto which Earth’s terrestrial north and south poles are projected. All celestial objects are seen to lie on this sphere and are assigned coordinates. The position of an object on the celestial sphere is given as its Right Ascension and Declination (Bennett, 1979). A representation of the celestial sphere is given in Figure 1.0.










Fig. 1.0: Diagram of the Celestial Sphere (Mosley, 1999)


Right Ascension is the is “...the equatorial distance between a point of reference, the First Point of Aries (γ), and the meridian of S (the celestial object)...” (Roelofs, 1950, p.5). The units of Right Ascension are hours. While declination is “...the spherical distance between S (the celestial object) and the equator...is plus or minus for north or south declinations respectively...” (Roelofs, 1950, p.6). The units of Declination are degrees.


There are different time systems in use, from the common mean solar time to mean sidereal time. Solar time is determined from the movement of the sun through the ecliptic. A solar day begins when the Sun passes through the lower branch of the Greenwich meridian (Bennett, 1979). The gnomon and sundial are used to determine solar time. This is further refined into mean solar time, which sees the movement of the sun as constant and it is this time that most people conduct their lives by.


Sidereal time is determined from the movement of stars. This time system relies on the assumption that the Earth rotates at a constant rate, that the orbit of the Earth will not produce any obvious effect on the stars position, then a star will complete a rotation at a constant rate, fixing the passage of time (Mackie, 1971). Then from this, if the distance from the local observer to the first point of Aries is known then the local sidereal time can be determined (Mackie, 1971).




Last Update 29/10/2008