Introduction to the Sky "Down Under"
Suppose that you live in Charleston, South Carolina. Imagine that you go to bed, and in the morning, you awaken in Sydney, Australia, and do not know that you have been transported to a location south of the equator. It is a clear morning. The Sun appears to rise normally enough, but after awhile you notice something strange. Rather than progressing generally south and west, as the Sun always does during the early morning hours in the northern hemisphere, the Sun moves north and west. At high noon it sits squarely in the northern sky. The weather is improbable, everyone has a strange accent, and the phone numbers are weird. You look at the cover of the phone book or tune into a local radio station, and the mystery is solved—except, of course, for how you got transported halfway around the world without remembering any of the trip.
If you normally live in Sydney and some morning you awaken in Charleston, a similar surprise awaits you. In fact, if you come from “down under” and are transported to America by surprise, you’ll be every bit as jarred as an American who is transported to Australia.
Southern Coordinates
Southern celestial coordinates are similar to northern celestial coordinates. They operate according to the same mathematics. The main difference is that the two coordinate hemispheres are mirror images of one another. While the northern heavens seem to rotate counterclockwise around the north celestial pole, the southern Sun, Moon, planets, and stars seem to rotate clockwise around the south celestial pole.
Recall your middle-school algebra class. Imagine the cartesian coordinate plane. The northern hemisphere is akin to the first and second quadrants, where the y values are positive; the southern hemisphere is cousin to the third and fourth quadrants, where the y values are negative. No particular quadrant is preferable to or more special than any of the other three. So it is with Earth and sky. Fully half the points on the surface of Earth are south of the equator. It is no more unusual in this world for the Sun to shine from the north at high noon than it is for the Sun to shine from the south. Only the extreme polar regions experience conditions that most people would call truly strange, where the Moon or Sun can stay above the horizon for days or weeks at a time, circling the points of the compass.
Southern Az/el
In the southern hemisphere, azimuth bearings are measured clockwise with respect to geographic north, just as they are in the northern hemisphere. However, an alternative system can be used; azimuth can be defined as the angle clockwise relative to geographic south. In this latter system, the range of possible values is from 0 degrees (south) through 90 degrees (west), 180 degrees (north), 270 degrees (east), and up to, but not including, 360 degrees (south again). This is shown in Fig. 3-1. The bearing of 360 degrees is omitted to avoid ambiguity, just as is the case with the north-based system. You will never hear of an azimuth angle less than 0 degrees nor more than 360 degrees, at least not in proper usage.
Figure 3-1 . Azimuth based on a southerly point of reference.
The elevation of an object in the sky is the angle, in degrees, subtended by an imaginary arc running away from the object until it intersects the horizon at a right angle. For visible objects over flat terrain, this angle can be as small as 0 degrees when the object is on the horizon or as large as 90 degrees when the object is directly overhead. This is exactly the same scheme as is used in the northern hemisphere. Elevation bearings are measured upward from the horizon to 90 degrees and downward below the horizon to –90 degrees. If the horizon cannot be seen, then it is defined as that apparent circle halfway between the zenith and the nadir.
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.
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.
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.
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.
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.
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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