The Celestial
Sphere

Much of our initial discussion of Astronomy will concern the motion of objects in the sky. Therefore, we shall introduce some terminology and a coordinate system that allow us to specify succinctly the location of particular objects in the heavens. For a more extensive discussion, see Astronomy without a Telescope.

The Celestial Sphere

It is useful in discussing objects in the sky to imagine them to be attached to a sphere surrounding the earth. This fictitious construction is called the celestial sphere. At any one time we see no more than half of this sphere, but we will refer loosely to the imaginary half-sphere over our heads as just the celestial sphere (see adjacent figure).

The point on the celestial sphere that is directly over our heads at a given time is termed the zenith. The imaginary circle passing through the North and South points on our horizon and through the zenith is termed the celestial meridian. We will introduce additional terminology associated with the celestial sphere later.

Motion in the Sky

It is clear after only minimal observation that objects change their position in the sky over a period of time. This motion is conveniently separated into two parts:
  1. The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours. This is termed the daily or diurnal motion of the celestial sphere, and is in reality a consequence of the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their relative positions: the diurnal motion causes the sky to rotate as a whole once every 24 hours.

  2. Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon.
Diurnal motion

Diurnal motion at different latitudes

Actually, all objects are slowly changing their relative positions on the celestial sphere, but for most the motion is so slow that it cannot be detected over timespans comparable to a human lifetime; only the "wanderers" have sufficiently fast motion for this change to be easily visible.

Celestial Coordinate Systems

We can define a useful coordinate system for locating objects on the celestial sphere by projecting onto the sky the latitude-longitude coordinate system that we use on the surface of the earth. As illustrated in the adjacent figure, this allows us to define "North and South Celestial Poles" (the imaginary points about which the diurnal motion appears to take place) and a "Celestial Equator".

The figure illustrates that these imaginary objects are the exact analogs of the corresponding imaginary objects on the surface of the earth. Thus, we shall be able to specify the precise location of things on the celestial sphere by giving the celestial analog of their latitudes and longtitudes, or something related to those quantities.

The "Road of the Sun" on the Celestial Sphere

Another important imaginary object on the celestial sphere is the "ecliptic" or "Road of the Sun", which is the imaginary path that the Sun follows on the celestial sphere over the course of a year. As the diagram at left indicates, the apparent position of the sun with respect to the background stars (as viewed from Earth) changes continuously as the Earth moves around its orbit, and will return to its starting point when the Earth has made one revolution in its orbit.
Thus, the Sun traces out a closed path on the celestial sphere once each year. This apparent path of the Sun on the celestial sphere is called the ecliptic. Because the rotation axis of the Earth is tilted by 23.5 degrees with respect to the plane of its orbital motion (which is also called the ecliptic), the path of the Sun on the celestial sphere is a circle tilted by 23.5 degrees with respect to the celestial equator (see diagram at right).

The ecliptic is important observationally, because the planets, the Sun (by definition), and the Moon are always found near the ecliptic. As we shall see later, this is because all of these objects have orbits that lie nearly in the same spatial plane.

Shockwave movie illustrating the ecliptic.

Motion of Sun through Zodiac in Copernican Model.

East and West on the Celestial Sphere

It is useful to define east and west directions on the celestial sphere, as illustrated in the following figure.

Thus, objects to the west of the Sun on the celestial sphere precede the Sun in the diurnal motion of the celestial sphere (they "rise" before the Sun and "set" before the Sun). Likewise, objects to the east of the Sun trail the Sun in the diurnal motion (they "rise" after the Sun and "set" after the Sun). Generally, one object is west of another object if it "rises" before the other object over the eastern horizon as the sky appears to turn, and east of the object if it "rises" after the other object.

Venus as the Morning Star.

Venus as the Evening Sstar.

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