The alert reader will notice that Figures 2.1 and 2.2 are crisscrossed by lines that look like latitude and longitude on a globe.
That's exactly what they are, but the globe to which they refer is the celestial sphere (Figure 2.3). This is an imaginary sphere, infinitely large, surrounding the Earth, on which the stars have fixed positions. Since the sphere is imaginary, its motion can be imaginary too, so astronomers pretend that the Earth holds still (with the observer's location right on top, of course) and the sphere rotates around it. Its rotation is that of the Earth, but in the opposite direction.
Coordinates on the celestial sphere are called declination (abbreviated Dec. or 5) and right ascension (R.A., AR, or a). Together, right ascension and declination are known as equatorial coordinates because they are based on a sphere whose equator and poles correspond to those of the Earth.
Declination is like latitude and is measured in degrees, negative if south of the equator. Right ascension is like longitude but is measured in hours (0 to 24). Since the sphere rotates once per day, it makes sense to measure longitude in time units.
Figure 2.4 shows the whole sky (as seen at the same time and place as Figures 2.1 and 2.2) with lines of R.A. and declination labeled.
Figure 2.4. The whole sky as seen in Figures 2.1 and 2.2. Based on a chart created with Starry Night Pro astronomy software (http://www.starrynight.com), reproduced by permission.
Hint: In astronomy, the directions north, south, east, and west always refer to the celestial sphere unless otherwise indicated. To move north in the sky means to move toward the north celestial pole.
The right ascensions and declinations of the stars are, for all practical purposes, constant. The Sun, Moon, and planets move around on the celestial sphere, so their right ascensions and declinations vary.
The term right ascension sounds as if it refers to something ascending or rising at a right angle, and indeed it does. Seen from the Earth's equator, all celestial objects rise and set at right angles to the horizon, at times that depend directly on their right ascensions. Declination is from the Latin word for "bending", an appropriate name for an angle.
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