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Right Ascension and Declination Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 15, 2011

The Sun’s Annual “Lap”

As the days pass during the springtime, the Sun stays above the horizon for more and more of each day, and it follows a progressively higher course across the sky. The change is rapid during the early springtime and becomes more gradual with approach of the summer solstice , which takes place on June 22, give or take about a day.

At the summer solstice, the Sun has reached its northernmost declination point, approximately dec = +23.5 degrees. The Sun has made one-quarter of a complete circuit around its annual “lap” among the stars and sits at RA = 6 h. This situation is shown in Fig. 1-8 using the same two az/el coordinate schemes as those in Fig. 1-7. The gray line represents the Sun’s course across the sky. As in Fig. 1-7, the time of day is midafternoon. The observer’s geographic latitude is the same too: 39°N.

Coordinating the Heavens Right Ascension And Declination

Figure 1-7. Az/el sky maps for viewer lying flat, face-up. At A , top of head facing north; at B , top of head facing south.

Coordinating the Heavens Right Ascension And Declination The Sun’s Annual “lap”

Figure 1-8. Az/el sky maps for midafternoon at 39 degrees north latitude on or around June 21.

After the summer solstice, the Sun’s declination begins to decrease, slowly at first and then faster and faster. By late September, the autumnal equinox is reached, and the Sun is once again at the celestial equator, just as it was at the vernal equinox. Now, however, instead of moving from south to north, the Sun is moving from north to south in celestial latitude. At the autumnal equinox, the Sun’s RA is 12 h. This corresponds to 180 degrees.

Now it is the fall season in the northern hemisphere, and the days are growing short. The Sun stays above the horizon for less and less of each day, and it follows a progressively lower course across the sky. The change is rapid during the early fall and becomes slower and slower with approach of the winter solstice , which takes place on December 21, give or take about a day.

At the winter solstice, the Sun’s declination is at its southernmost point, approximately dec = –23.5 degrees. The Sun has made three-quarters of a complete circuit around its annual “lap” among the stars and sits at RA = 18 h. This is shown in Fig. 1-9 using the same two az/el coordinate schemes as those in Figs. 1-7 and 1-8. The gray line represents the Sun’s course across the sky. As in Figs. 1-7 and 1-8, the time of day is midafternoon. The observer hasn’t moved either, at least in terms of geographic latitude; this point is still at 39°N. (Maybe the observer is in Baltimore now or in the Azores. Winter at Lake Tahoe can be rough unless you like to ski.)

Coordinating the Heavens Right Ascension And Declination The Sun’s Annual “lap”

Figure 1-9. Az/el sky maps for midafternoon at 39 degrees north latitude on or around December 21.

After the winter solstice, the Sun’s declination begins to increase gradually and then, as the weeks pass, faster and faster. By late March, the Sun reaches the vernal equinox again and crosses the celestial equator on its way to warming up the northern hemisphere for another spring and summer. The “lap” is complete. The Sun’s complete circuit around the heavens takes about 365 solar days plus 6 hours and is the commonly accepted length of the year in the modern calendar. In terms of the stars, there is one extra “day” because the Sun has passed from west to east against the far reaches of space by a full circle.

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