Far South on the Sphere

Date: April 1-15
Time: 7:45 pm, one hour after sunset
Place: the southern sky

You probably know there are stars and constellations you can see from south of the equator, and you can’t see from up here in the northern hemisphere. The Southern Cross is a well known example. So just how far south can you see?

Think of the night sky as a huge sphere, with Earth at its center (Aristotle did). In Aristotle’s model, which is incorrect but still useful, the stars are fixed to this sphere, and rotate in unison around the Earth. Polaris, our North Star is located at the north end of the sphere’s axis of rotation. Like latitude on a globe, we measure north and south on this celestial sphere by declination.

Polaris has declination +90°; the South Celestial Pole (there is no “South Star” down under) has declination -90°. Depending on your latitude, you can see more or less of the north and south halves of the sky. The formula:  D = L – 90° tells you just how far south you can see. D is the declination, and L is your latitude.

Three bright stars, far south on the sphere, are now within visible range. They attain their highest elevations as they cross the meridian, the north-south line in the sky. The arcing path of ζ Puppis (shown) brings it up to a maximum elevation of 16°. If your skies are clear and dark, you just might spot it. The other two stars, at elevations 13° and 9°, are a much greater challenge.

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