Excerpts from Jim Conrad's
Naturalist Newsletter

Southern Cross

from the January 3, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
THE SOUTHERN CROSS

You may be surprised to learn that from atop the little hill where I stay, when I go out to jog before the sun comes up each day, I can clearly see the constellation known as the Southern Cross, or Crux Australis, hanging right above the horizon exactly to the south.

We're 20 degrees north of the Equator here, at about N20°40', so how come I can see the Southern Cross, which we think of as marking the spot around which all the rest of the southern sky rotates, at 90°00' south of the celestial equator? Also you may be surprised that after looking at the Southern Cross I can turn around and very clearly see the North Star, Polaris, suspended above the northern horizon about a hand's width high if the hand is held at arm's length.

The secret to this riddle is that, while Polaris does lie pretty close to the north celestial pole at 90°00' (Polaris is at 89°18'), the Southern Cross stands a good 20° away from the south celestial pole. In fact, the South 60° line runs right through the constellation.

One thing that this means is that the Southern Cross isn't nearly as precise as a direction giver as the North Star. It also means that six months from now the Southern Cross will be visible from here around dusk but not at dawn. If I'm here jogging at dawn next June, the Southern Cross will lie way below the horizon.

If I can see the Southern Cross from the Yucatan, can people in the US see it? From Miami theoretically the Southern Cross's four stars lie above the horizon. However, any clouds or haze on the distant horizon would obscure the constellation, or at least its lower stars. As far north as Atlanta, Georgia, however, already the Cross lies completely below the horizon.

Here's how I locate the Southern Cross here. First I find the Big Dipper, Ursa Major. Letting Ursa Major's handle point across the sky I look for a bright star about 1 hand-width and three finger-widths away. That bright star is Arcturus in the constellation Boötes. Keep following the line in the same direction for another one-hand-and-three-finger-widths and you come to a similarly bright star, which is Spica in the constellation Virgo. (Remember that from the Big Dipper's handle tip you "Arc to Arcturus, then spike to Spica... "). Continue on the same trajectory but this time for about two hand-widths, almost to the horizon, but jag maybe half a hand-width to the left.

There you see four stars right above the horizon, the one on the far right not as bright as the others, and those stars are arrayed like the four points of a kite, or a cross without a center spot. That's the Southern Cross.

On my computer I have an old freeware sky-observation program called SpaceExplorer. It generates star and planet charts for any date, time and place. The screen showing the Southern Cross, labeled Crux Australis at the center bottom, as seen here at 5 AM on January 1, is at the top of this page.

In that picture the green, curved line indicates my horizon, so you can see how far above the horizon the Southern Cross is when seen from here. In the US and Europe you should be able to see Leo and Virgo at dawn. The red dot in Leo is Mars and the yellow symbol in Virgo is Saturn.