The clarity of the night sky. A low number means clear, black and amazing stars. Higher numbers mean gradually more urban environments with increasing light pollution. At 9 you’ll barely spot a star being pretty much in Piccadilly Circus or Times Square.

More detail needed? John Bortle, Light Pollution And Astronomy: The Bortle Dark-Sky Scale, Sky and Telescope, July 18 2006.

Bortle Scale

Where the Northern Hemisphere simply allows you to find the North Star, around which all the stars will rotate, finding your way in the Southern Hemisphere is a little more involved. Unlike in the North, there’s no obvious star due south in the Southern Hemisphere. Instead, you have to use two handy indicators: the Southern Cross — easily spotted, fortunately — and two stars known as The Pointers. Find where these intersect, as in the sketch, and you’ve found due south.

Or, if you prefer, you can follow the direction the Southern Cross is pointing for a distance equal to 4.5x the length of the Southern Cross to arrive at the spot.

Incidentally, I wondered about reliable methods of finding south in the Northern Hemisphere and north in the Southern Hemisphere. And I think the answer is to find north or south and turn around.

Sketchplanations tagged with stars