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Suppose you live in a mid-northern latitude. Specifically suppose you live in the northeast US. You can see at most down to a declination of maybe 50° South, but let's call it 45°S since the stars down near the horizon are often lost due to atmospheric extinction. That means that you can readily learn the sky from 90°N dec down to 45°S dec. Hmmm... sounds like 75% of the celestial sphere, and it seems to imply that you have 25% more to learn if you hope to know your way around the southern sky. But it's not so bad! First of all, because of the behavior of coordinates on the "sphere", that fraction below 45°S is smaller than it sounds. It's less than 15% of all directions, all possible locations on the celestial sphere. The fraction can be calculated from the "polar distance" angle away from either celestial pole (or any arbitrary point, too):
   frac = [1 - cos(p.d.)] / 2,
or if you like,
   frac = haversine(p.d.).

In addition to this simple geometric factor, when you start working your way south, you'll discover the "Celestial South Hole" --that barren wasteland with no bright stars. It starts at about 70°S and continues to the SCP, the South Celestial Pole, which famously has no "Southern Star". Rather than needing to learn the whole sky from 45°S and on down, you find that you only have to worry about a nice manageable "belt" around the celestial sphere from 45°S to 70°S, and that is less than eight percent of the entire celestial sphere. Not so bad, right?

Putting it a little differently, 85% of the celestial sphere is already available to you to learn by direct observation from a mid-northern latitude. You need to add the 8% band to that so it's less than 10% of what you already see. Just work your way around that "belt" in good star charts or in a simulation app like "Stellarium". Start with alpha and beta Centauri, then Crux, the southern cross, then pick up Miaplacidus and the false cross, followed by Canopus and swinging far around to Achernar and through a few other bright landmark stars like Atria to complete the circle. Naturally, the southern sky will not be recognizable like the back of your hand the way the northern sky might be, but you'll have the basemap to fill in easily when you see photos of the southern sky ...or when you can manage to get south far enough for the real thing.

And me? I'm still looking forward to a trip to a latitude of at least 15°N and better yet 15°S... That northern margin, 15°N, would be far enough to see that key "belt" of stars and maybe in time to watch eta Carinae go supernova! :)

Frank Reed

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