Astro Bob: 2,008 miles to the Southern Cross
As winter revisits northern Minnesota with wind-whipped snow my thoughts turn southward.
DULUTH — I can't see the Southern Cross from my house. Not by a long shot. Chances are you can't see it either. Unless you live or travel south of 25 degrees north latitude it never pokes a beam above the horizon. That's not to say it's invisible from the U.S. In Key West, just 2,008 miles from my front door, the kite-shaped constellation, formally called Crux, stands upright and highest around midnight in mid-April.
High is a relative term. Acrux, the bottom-most star in the cross, hunkers down in the haze just 2.5 degrees above the horizon, so low that distant clouds can block it from view. But on an ideal night, when a soft Gulf breeze ruffles your hair, and the sky shimmers with stars, Crux and the southern Milky Way that garbs it are captivating sights.
Of the 88 constellations the Southern Cross is the smallest. But that doesn't mean it takes a back seat. Three of the four stars that outline its iconic shape shine at first magnitude and pack a lot of visual punch. Also, the cross sits smack in the middle of two even-brighter stars — Alpha and Beta Centauri — and the Eta Carina Nebula. Like the Orion Nebula, Eta Carina is a massive stellar nursery but much larger and more luminous. It looks like a bright, fuzzy "star" with the naked eye.
The ancient Greek astronomers of circa 300 B.C. knew about Crux but instead of a cross they pictured it as the hind legs of Centaurus the centaur rather than a unique constellation. Due to the slow wobble of Earth's axis called precession — the same wobble that causes the polestar to change over thousands of years — Crux slowly slipped southward in that era. By around the year 800 A.D. it was no longer visible from mid-northern latitudes and consequently forgotten.
Crux was rediscovered during the Age of Exploration (1400-1600s), when Europe navigators set sail across the globe. Those who saw it likened the compact group to a cross and used it as a navigational aid — Crux points to the south celestial pole and polestar the same way the two stars in the bucket of the Big Dipper point to Polaris.
Crux first appeared on celestial globes created by Dutch cartographers in 1598 and 1600. English explorer Robert Hues, who saw the cross on two southern voyages in the late 1500s, came to realize that the figure's stars were the same the Greeks assigned to the centaur's hind legs. With this knowledge in hand, astronomer Johann Bayer drew the outline of the cross in his 1603 Uranometria atlas but placed the stars in Centaurus.
Crux was first listed as a separate constellation called "De Cruzero" in a Dutch catalog that same year, but it wasn't until 1624 that it finally appeared in a printed atlas with its own unique identity as the Southern Cross. In the modern era the iconic form has become symbolic of the southern hemisphere and appears on the flags of five nations — Australia, New Zealand, Brazil, Samoa and Papua New Guinea.
Crux and the neighboring southern Milky Way are a focal point for southern observers the same way Orion and the Summer Triangle are beloved in the northern hemisphere. Five stars of magnitude 1 or brighter shine in a small patch of sky not far from the sky's brightest nebula and also one of its darkest, a prominent "hole" in the Milky Way called the Coalsack Nebula. The Milky Way in the Crux-Carina region is not only bright and chunky-looking with the naked eye but full of star clusters, some of which are easily visible in binoculars.
Just writing about the Southern Cross makes me want to get down to Florida ASAP. Or better, go a little further south to the Caribbean or South America to witness its full glory unfiltered by low-altitude haze. When I traveled to Antarctica and the Falkland Islands a year and a half ago, Crux along with Alpha and Beta Centauri were my touchstone stars. They helped me get my bearings in a sky filled with new constellations that seemed to move in the "wrong" direction.
If you're like me and can't hop on a plane just this moment, we can at least picture where Crux will be the next clear night. Face south around 10 p.m. local time. High up you'll spot Leo the lion and the star Regulus. If you lower your gaze and look southeast you'll see another bright star. That's Spica in Virgo. Just to the lower right of Spica look for a small trapezoid of stars. That's Corvus! I encourage you to familiarize yourself with this part of the spring sky so you can be ready when the time comes to fly south.