Southern Coordinates Help (page 2)

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

Southern Ra/dec

There are two points in time every year when the Sun’s elevation, measured with respect to the center of its disk, is positive for exactly 12 hours and negative for exactly 12 hours. One of these time points occurs on March 21, give or take about a day; the other occurs on September 22, give or take about a day. At the equinoxes, the Sun is exactly at the celestial equator; it rises exactly in the east and sets exactly in the west, assuming that the observer is not at either of the geographic poles. The names vernal and autumnal , as used in the northern hemisphere, are not really correct in the southern hemisphere because the seasons are reversed compared with those in the north. Thus it is best to speak of the March equinox and the September equinox .

The crude celestial maps of Fig. 3-2 show the situation at either equinox. That is, the date is on or around March 21 or September 22. You can deduce this because the Sun rises exactly in the east and sets exactly in the west, so it must be at the celestial equator. At the latitude of Sydney, the Sun is 35 degrees away from the zenith (55 degrees above the northern horizon) at high noon on either of these days. The south celestial pole, which unfortunately has no well-defined sentinel star, as is the case for the northern hemisphere, is 35 degrees above the southern horizon all the time. The heavens seem to rotate clockwise around the south celestial pole. In the drawing at A , imagine yourself lying flat on your back, with your head facing north and your feet facing south. In the drawing at B , imagine yourself rotated 180 degrees, that is, with your head facing south and your feet facing north. Either orientation is valid astronomically, and you will find star maps that use either scheme.

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.

Every day the Sun moves slightly toward the east with respect to the background of stars. At the March equinox, the Sun is at the celestial equator and is located in a certain position with respect to the stars. This represents the reference point for right ascension (RA) and declination (dec). As time passes, the Sun rises about 4 minutes later each day relative to the background of stars. The sidereal (star-based) day is about 23 hours and 56 minutes long; the synodic (sun-based) day is precisely 24 hours long. In the southern hemisphere, the Sun’s motion relative to the stars is from left to right.

In the drawings of Fig. 3-2, the Sun is at dec = 0 degrees. Suppose that these drawings represent the situation at the March equinox. This point among the stars is the zero point for right ascension (RA = 0 h). As autumn passes and the Sun follows a lower and lower course across the sky, the declination and right ascension both increase for a while. Remember that right ascension is measured in hours, not in degrees. There are 24 hours of right ascension in a circle, so 1 hour (written 1 h or 1 h ) of RA is equal to 15 degrees.

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