Saturday, December 4, 2021

The Southern Cross



By Mike P. Usher

We are going to change our viewing time from our usual 9PM to 11PM until October. There are two reasons for this: 1) The longer days of summer mean it is not fully dark at 9PM, and 2) the rapidly approaching rainy season means early evening clouds blot out the stars. By 11PM there is at least a chance of clearing, although it’s not usually totally clear until after midnight.

Did you think you would have to travel to Argentina to see the Southern Cross? Not so! Theoretically, you can see the whole cross (official name is Crux) right here on Marco, although you have to work fairly hard at it. The main problem is the southernmost star of the cross, Acrux. It does rise above the horizon, but just barely. Only about one-half of a degree at the highest point, which occurs at 10:30PM. You will certainly need binoculars, a completely flat horizon (i.e. the Gulf), and a little bit of altitude to help get your eye above the horizon mark. Possibly use one of the hills on the island, if you can get a clear southern view, or perhaps the top of a condo. I’ve even been told a stepladder helps! Incidentally, Crux is the smallest of all of the constellations – your palm held at arm’s length could cover all of it.

Also in the deep south is the star named Rigel Kent, much better known by it’s Bayer catalog designation as Alpha Centauri, the closest naked eye visible star to the Sun. It’s the third brightest star in the heavens when it is high in the sky; but you will still have trouble spotting Alpha Centauri without binoculars, as it too is very close to the horizon, although not as bad as Acrux. Try looking for it at midnight or 1:00 AM, when it is at its highest.

Easier and more interesting to spot is Omega Centauri, the largest globular star cluster visible from Earth. Greater in apparent diameter than the full Moon, it contains about one million stars. It is so different from other globular clusters there is a debate that it might actually be the remaining core of an absorbed dwarf galaxy. Faintly visible to the naked eye, it appears dimmer than its given magnitude of 3.7 as it is more “spread out” than a star. Through binoculars Omega Centauri looks like a ball of cotton, a large telescope will resolve the cluster into individual stars. Omega Centauri is abbreviated on the accompanying chart as ? Cen.

See you next time!


Mr. Usher is president of the Everglades Astronomical Society, which meets the second Tuesday each month at 7 PM in the Norris Center at Cambier Park in Naples.

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