Southern Coordinates Help

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

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.

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

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.

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