Apparent Daily Motion Of The Stars

Although the North Star, Polaris, is not a very bright star, it has long been important for navigation. Closest to the north celestial pole, it is the only star that seems to stay in the same spot in the sky. You can find Polaris by following the "pointer stars," Dubhe and Merak, in the bowl of the Big Dipper in the constellation Ursa Major (Figure 1.8).

Since the celestial poles are at distinct altitudes in the sky at distinct latitudes, the part of a star's diurnal circle that is above the horizon is different at different latitudes on Earth (Figure 1.9).

For example, if you stargaze at 40°N latitude, about the latitude of Denver, Colorado, U.S., you will see (Figure 1.9): (1) Stars within 40° (your latitude) of the north celestial pole (those stars between +50° and +90° declination) are always above your horizon. These stars that never set—such as the stars in the Big Dipper—are north circumpolar stars. (2) Stars that are

North Star

Figure 1.8. The "pointer" stars, Dubhe and Merak, in the bowl of the Big Dipper lead you to the North Star, Polaris. The angular distance between these pointer stars is about 5° on the celestial sphere. A fist at arm's length marks about 10°. These examples will help you judge other angular distances in the sky.

Figure 1.8. The "pointer" stars, Dubhe and Merak, in the bowl of the Big Dipper lead you to the North Star, Polaris. The angular distance between these pointer stars is about 5° on the celestial sphere. A fist at arm's length marks about 10°. These examples will help you judge other angular distances in the sky.

Figure 1.9. The sky from 40°N latitude. The north celestial pole is 40° above the northern horizon, and the celestial sphere rotates around it. Parallels of declination mark the stars' diurnal circles.

within 40° (your latitude) of the south celestial pole never appear above your horizon. These stars that never rise—such as the stars in the constellation Crux, the Southern Cross—are south circumpolar stars. (3) The other stars, in a band around the celestial equator, rise and set. Those stars that are located at 40°N declination (equal to your latitude) pass directly across your zenith when they cross your celestial meridian.

Assume you are stargazing at 50°N latitude, about the latitude of Vancouver, Canada. Refer to Table 1.1 for the declinations of the bright stars Capella, Vega, and Canopus. Which of these stars will be above your horizon: (a) always?_(b) sometimes?_(c) never?_

Answer: (a) Capella (+46°00' declination). Stars within 50° of the north celestial pole (between +40° and +90° declination) are always above the horizon. (b) Vega (+38°47' declination). This star rises and sets. (c) Canopus (-52°42' declination) is within 50° of the south celestial pole (between -40° and -90° declination).

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