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This sky map shows the location of Ursa Major, the constellation containing the well known Big Dipper star pattern, as it appears in the late April night sky at mid-northern latitudes. Credit: Starry Night Software |
After the sun has set and the sky gets dark on these early spring evenings, what might be the most familiar pattern of stars for a stargazing neophyte to recognize? If you live north of the equator, you need only look toward the northern part of the sky, where you will find the seven bright stars that comprise the famous Big Dipper.
When I was starting out in astronomy as a young boy living in the Bronx, I had taken a book out of my local library — "The Boys Book of Astronomy," by Patrick Moore. But in Chapter 9 ("The Stars of Spring"), I became confused briefly, because Moore referred to the seven stars of the Big Dipper as "The Plough." It wasn’t until later that I realized that Moore is a revered British astronomer, and that in the British Isles those seven stars are widely known as The Plough.
For most skywatchers, the Big Dipper (or Plough) is probably the most important group of stars in the sky. For anyone in the latitude of New York (41 degrees North) or points northward, it never goes below the horizon. It is one of the most recognizable patterns in the sky and thus one of the easiest for the novice to find.
The Southern Cross
But if you live south of the equator, it’s not the Big Dipper that people choose as their guide to the night sky. Rather, it’s the constellation known as Crux, or the Southern Cross. Those in the Southern Hemisphere (where it is now early autumn) need only cast a glance toward the south where they’ll see the distinctive shape of the Cross hanging well up in the sky.
Although known as a cross, it really looks more like a kite, clearly outlined by four bright stars. Two of those four, Acrux and Becrux, are first-magnitude stars, ranking among the 21 brightest in the sky. From top to bottom, Crux measures just 6 degrees — only a little taller than the distance between the pointer stars of the Big Dipper. [Best Beginner Astrophotography Telescopes]
In fact, the Southern Cross is the smallest (in area) of all the constellations. It is also honored on the flags of such countries as Australia and New Zealand, and its image is found on numerous postage stamps.
It is thought that Amerigo Vespucci was the first European voyager to see the "Four Stars," as he called them, while on his third voyage in 1501. But Crux was actually visible everywhere in the United States some 5,000 years ago, as well as in ancient Greece and Babylonia.
According to Richard Hinckley Allen (1838-1908), an expert in stellar nomenclature, the Southern Cross was last seen on the horizon of Jerusalem about the time that Christ was crucified. Thanks to precession — an oscillating motion of the Earth’s axis — over the centuries, the Cross gradually got shifted out of view well to the south.
Credit: Starry Night
Viewing the iconic constellations
There are probably a number of you who have never seen either the Big Dipper or the Southern Cross and might wonder about how far you might have to travel in order to get a view of them. Coincidentally, at this time of the year, both are attaining their highest positions in the sky at the same time: just before midnight local daylight time.
To see Crux, one must go at least as far south as latitude 25 degrees North. In the continental United States, that means heading to the Florida Keys, where you’ll see it just lifting fully above the southern horizon. A slightly better view is afforded to those living in Hawaii, where the Cross appears several degrees higher.
For Southern Hemisphere dwellers who want to see the Big Dipper, you must go north of latitude 25 degrees South to see it in its entirety. Across the northern half of Australia, for instance, you can now just see the upside-down Dipper virtually scraping the northern horizon about an hour or two after sundown. In fact, it’s the opposite effect as that observed by people who live in north temperate locations like New York. They see the Dipper at a similar altitude above the northern horizon on early evenings in late November or early December — except the Dipper appears right-side up!
Like the Big Dipper, whose Pointer stars point toward Polaris (the North Star), the Southern Cross indicates the location of the South Pole of the sky and as such is often utilized by navigators. The longer bar of the Cross points almost exactly toward the South Pole of the sky, which some aviators and navigators have named the "south polar pit" because, unfortunately, it is not marked by any bright star.
But don’t be fooled. Those who live south of the equator are probably well acquainted with four other bright stars nearby. Two belong to the constellation Vela (the Sails) and two belonging to Carina (the keel of the great ship Argo); they're known collectively as the "False Cross." [10 Skywatching Misconceptions Explained]
Not only do these four stars bear a superficial resemblance to the true Southern Cross, but they're even oriented in roughly the same way (though positioned a bit farther to the north). So an uninitiated observer might easily mistake the X-shaped False Cross for the true one, but only the latter points to the south celestial pole.
Yet the False Cross will have its time of glory in the year 8600, when the pole will have shifted to a point just east of the cross-arms’ intersection. By then, perhaps when our descendants are asked where the South Pole of the sky is located, they can just say, "X marks the spot."
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
