Time, Sidereal

Observational Effects

A less stringent method can be applied to any object on the celestial sphere, for example an arbitrary star such as Antares (Figure 2a). Any star appearing against the stellar background will need one complete sidereal day to return to its starting point (Figure 2b), as Antares has below. This includes not only stars near the earth's equator, but also stars and objects that appear at both polar regions and all points between. Since the sun is comparatively close to the earth, and the distant star is relatively “fixed” to the stellar background, the sun will take an additional 3 minutes, 56 seconds of the earth's rotational time to return to the local meridian than does the star. While the earth's axis is currently directed toward the region of Polaris, it is not fixed, and Polaris on a daily basis exhibits a small circular path that takes one sidereal day to complete. Stars near the poles tend to circle the polar regions visibly night to night, and are called circumpolar stars. These stars neither rise nor set because the earth's axis of rotation is directed toward those regions.

Precession of the Poles

The earth's northern polar axis will not always remain constantly aimed at Polaris, however (and the southern axis will change accordingly). The earth is acted upon by external gravitational forces such as the sun, moon, and other planets. This effect causes the earth to wobble and not spin vertically, but rather obliquely, like a gyroscope. This effect is called precession, and influences where the earth's axis will celestially point in a somewhat large area of space. The cycle, or Precession of the Poles, takes approximately 25,800 years to complete. In approximately 12,000 years from now, the star Vega will be the object of direction for the earth's northern axis. The South Star is currently Sigma Octans, and is quite faint. However, in 2,200 years, the star Gamma Chameleon will mark the southern pole with a bit more luminescence.

Figure 2a View of the sky at local noon on Day I. Both the sun and the star are on the local

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