Southern Coordinates Help (page 2)
Introduction to the Sky "Down Under"
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 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.
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.
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.
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.
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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