Tuesday, May 16, 2006

What they don't tell you about southern skies

Most amateur astronomers are aware of the fact that there are all kinds of constellations and objects that are visible only in the Southern Hemisphere—like the Eta Carinae nebula, the Southern Cross, and the Coal Sack. But what surprised me on a trip to New Zealand last year is that everything is upside down.

Think about it—to get to the southern hemisphere you travel down the earth's surface, past our equator and underneath the ecliptic. So all of a sudden the ecliptic is in the north, not the south, which means that the moon is never where you think it ought to be. But, more shocking, the moon is upside down! Here's the drill: if you're in New York looking at a first quarter moon crossing the meridian due south, the bright side is to the right (west) and the dark side is to the left (east). You start walking south until you get to the equator. Now the moon is directly overhead. If you keep walking south until you get to Chile, the moon starts to move toward the north. If you turn around and stare at the moon, the lit side is still facing west, but now that's to your left. What used to be the top of the terminator in New York is now the bottom.

And, naturally, all the constellations that are near the ecliptic—like Orion—are upside down, too.

When I was in New Zealand, my young cousins were telling me about the only constellation they knew, "the Pot." I had never heard of this, so I asked them to point it out to me one night. Come to find out, it's actually Orion. Only, Orion upside down with the head and shoulders pretty low in the sky. So all you see is the bottom of his tunic, upside down. Kind of like a pot.


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