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Southern Coordinates Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 16, 2011

The Sun’s Annual “Lap” In The South

Let us begin following the Sun during the course of the year starting at the March equinox. As the days pass during the months of April, May, and June, 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 in the first days after the equinox, and becomes more gradual with the approach of the June solstice , which takes place on around June 22 give or take a day. This might be called the “winter solstice,” but again, to avoid confusion with northern-hemisphere-based observers who call it the “summer solstice,” it is better to name the month in which it occurs.

At the June 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. 3-3 using the same two az/el coordinate schemes as those in Fig. 3-2. The gray line represents the Sun’s course across the sky. As in Fig. 3-2, the time of day is midafternoon. The observer’s geographic latitude is the same too: 35°S.

The Sky “Down Under” Southern Coordinates Southern Ra/dec

Figure 3-2. Az/el sky maps for midafternoon at 35 degrees south latitude on or around the March or September equinoxes. At A , top of head facing north; at B , top of head facing south.

The Sky “Down Under” Southern Coordinates The Sun’s Annual “lap” In The South

Figure 3-3. Az/el sky maps for midafternoon at 35 degrees south latitude on or around June 21.

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

Now it is the spring season in the southern hemisphere, and the days are growing long. 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 September and October and becomes slower and slower with the approach of the December solstice , which takes place on December 21, give or take a day.

At the December solstice, the Sun’s declination is at its southernmost point, approximately dec = –23.5 degrees. The Sun has gone through three-quarters of its annual “lap” among the stars, and sits at RA=18 h. This is shown in Fig. 3-4 using the same two az/el coordinate schemes as those in Figs. 3-2 and 3-3. The gray line represents the Sun’s course across the sky. As in Figs. 3-2 and 3-3, the time of day is midafternoon. The observer hasn’t moved either, at least in terms of geographic latitude; this point is still at 35°S.

The Sky “Down Under” Mirrored Myths Sky Maps

Figure 3-4. Az/el sky maps for midafternoon at 35 degrees south latitude on or around December 21.

After the December solstice, the Sun’s declination begins to increase gradually and then, as the weeks pass, faster and faster. By late March, the Sun reaches an equinox again and crosses the celestial equator on its way to forsaking the southern hemisphere for another autumn and winter. The “lap” is complete.

Mirrored Myths

The Greeks didn’t name the southern circumpolar constellations, but many of the star groups near the equator, as seen from “down under,” are the same ones that the Greeks made famous. The only difference is that they are all upside down.

Sky Maps

In this chapter, the general shapes of the better-known southern constellations are shown. To see where these constellations are in the sky from your location this evening, go to the Weather Underground Web site at the following URL:

 

Type in the name of your town and country, and then, when the weather data page for your town comes up, click on the “Astronomy” link. There you will find a detailed map of the entire sky as it appears from your location at the time of viewing, assuming that your computer clock is set correctly and data are input for the correct time zone.

Practice problems of this concept can be found at: The Southern Sky Practice Problems

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