Celestial Sphere

The celestial sphere was a very useful concept developed by early astronomers to create a consistent coordinate system for the heavens. It is an imaginary sphere of very large radius with Earth as its center and on which all observable celestial bodies are assumed projected.

The rotational axis of Earth intersects with the north and south poles of the celestial sphere. An extension of Earth’s equatorial plane cuts the celestial sphere and forms a great circle, called the celestial equator. Consequently, the celestial poles and celestial equator are analogs of the corresponding constructs on the surface of Earth. Astronomers can specify the precise location of objects on the celestial sphere by giving the celestial equivalents of their latitudes and longitudes. For an observer on Earth’s surface the point on the celestial sphere directly overhead is the zenith. Astronomers call an imaginary arc passing through the celestial poles and through the zenith the observer’s meridian. The nadir is the direction opposite the zenith. For an observer on Earth’s surface the nadir is the direction straight down to the center of the planet.

In the very widely used equatorial coordinate system the coordinates are the declination and the right ascension, and the fundamental reference circle is the celestial equator. The declination is the celestial sphere’s equivalent of latitude. The celestial equator has a declination of 0°, the north celestial pole a declination of +90°, and the south celestial pole a declination of –90°. Consequently, declination values expressed in positive values of degrees extend from the celestial equator to the north celestial pole, while declination values expressed in negative values of degrees extend from the celestial equator to the south celestial pole.

Like the declination, the right ascension is the celestial equivalent of terrestrial longitude. Astronomers can express right ascension in degrees, but more commonly they prefer to specify it in hours, minutes, and seconds. They do this because as Earth rotates, the sky appears to turn 360° in 24 hours, and that corresponds to 15° per hour. As a result, an hour of right ascension is 15° of sky rotation. Another important feature of the celestial sphere is the ecliptic plane. When astronomers use the ecliptic coordinate system, they can plot the direction to any star or other celestial body in two dimensions on the inside of this large imaginary sphere by using celestial latitude and celestial longitude.

Questions to Ponder:

• What is the purpose of using the Celestial Sphere in the coordinate system?
• With respect to the earth, what is the assumed radius of the Celestial sphere?