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April 2003 Feature: Cassiopeia & the Southern Cross


The constellations Cassiopeia and Crux (better known as the Southern Cross) are like flip sides of a coin. You'll never see them in the sky together. In fact, when one is beneath the horizon, behind the scenes, it has to wait for its counterpart constellation to get off stage, before it can enter the starry sky. If you live north of 35 degrees N. latitude, Cassiopeia always stays above the horizon -- meaning the Southern Cross can never climb above it. Yet south of 35 degrees S. latitude, the Southern Cross is the one that's always above while it's Cassiopeia that remains underneath.

But in the tropics and subtropics, Cassiopeia and the Southern Cross take turns entering and exiting the sky. When one constellation sets, the other rises. Furthermore, they always rise and set about-face of one another. When you see one set, the other rises 180 degrees in the opposite direction.

Springtime is the best season in our Northern Hemisphere for observing the Southern Cross, especially if you don't want to stay up till after midnight. Anyone venturing to the warmer climes at spring break might want to keep this in mind, as this could be your chance of catching this southern dignitary in the northern skies. Lucky for us in the Northern Hemisphere, the Big Dipper provides a ready roadmap to this southern signpost.

At nightfall, look high in the northern sky, to see the Big Dipper upside down over Polaris, the north star. Extend the arc of the Big Dipper handle counter-clockwise through two brilliant stars, Arcturus and Spica, and swing onward to a trapezoid of stars -- the constellation Corvus, the crow. (Click on the looking north and looking south star maps for handy reference.)

When Corvus perches due south, at its highest point for the night, it trumpets the Southern Cross' presence on center stage. If you're at the latitudes of southern Texas and Florida or further south, you'll see the Southern Cross due south, standing upright, and directly beneath of Corvus. At this juncture, Cassiopeia dips below the northern horizon.

Anywhere north of 35 degrees N. latitude, however, Cassiopeia stays above the horizon -- so consequently, the Southern Cross can't rise above it. At 30 degrees N. latitude, Cassiopeia half sinks below the northern horizon, while the Southern Cross half rises above the southern horizon -- in effect, making the partially-present constellations difficult if not impossible to see. But if need be, you can always use Corvus to envison the Southern Cross with your mind's eye!

If you have difficulty finding south, use the Big Dipper to locate Polaris, the north star. Anytime during the night, all you have to do is to turn about-face of Polaris -- and presto, that's south. Whenever Corvus' trapezium of stars shines due south (or directly opposite of Polaris), Cassiopeia falls to its lowest point in the north, and at the same time, the Southern Cross reaches its highest point in the south.

Corvus and the Southern Cross shine due south around midnight in late March and early April, and about ten o'clock (daylight savings: eleven o'clock) in late April and early May.

Now to find another southern treasure: the Omega Centauri star cluster!

copyright by Bruce McClure

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