Astronomers draw imaginary horizontal and vertical lines on the celestial sphere similar to the latitude and longitude lines on Earth. They use celestial coordinates to specify directions to sky objects.
The celestial equator is the projection of the Earth's equator out to the sky. Angular distance above or below the celestial equator is called declination (dec). Distance measured eastward along the celestial equator from the zero point, the vernal equinox, is called right ascension (RA). Right ascension is commonly measured in hours (h), with 1h = 15°.
Just as any city on Earth can be located by its coordinates of longitude and latitude, any sky object can be located on the celestial sphere by its coordinates of right ascension and declination.
Give the location of the star shown in Figure 1.5._
Answer: 20h RA, 30°N declination.
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