Doorstep Astronomy: See the Big Dipper
From northern latitudes, use the Big Dipper to point the way to Polaris and the north celestial pole. Currently, the Big Dipper can be seen at its highest in the northern sky late in the evening, with its bowl overturned.
As soon as darkness falls these evenings, step outside and look skyward. What is the most prominent and easiest star pattern to recognize? If you live in the Northern Hemisphere you only need to look overhead and toward the north where you will find the seven bright stars that comprise the famous Big Dipper.
For most sky gazers, the Big Dipper 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.
In other parts of the world, these seven stars are known not as a Dipper, but as some sort of a wagon. In Ireland, for instance, it was recognized as "King David's Chariot," from one of that island's early kings; in France, it was the "Great Chariot." Another popular name was Charles's Wain (a wain being a large open farm wagon). And in the British Isles, these seven stars are known widely as "The Plough."
Of greatest importance is the ability to utilize the Big Dipper to locate Polaris, the North Star. This is made possible by the two bright stars that mark the outer edge of the bowl of the Big Dipper. These two stars Dubhe and Merak are known as the "Pointers," because they always point to Polaris. Just draw a line, in your imagination, between these two stars and prolong it about 5 times the distance between these two stars and this line will ultimately hit a moderately bright star. That will be Polaris.
The Southern Cross
But for those who live in the Southern Hemisphere, it's not the Big Dipper that people choose as their guide to the night sky but rather, it's the constellation known as Crux, the Southern Cross. Those south of the equator (where the season is now midautumn), need only cast a glance toward the south where they'll see the distinctive shape of the Cross hanging well up in the sky. To some, it looks more like a kite, though the Cross is clearly outlined by four bright stars, two of which, Acrux and Becrux, are of the first magnitude. From top to bottom, Crux measures just 6 degrees only a little taller than the distance between the Pointer stars of the Big Dipper. In fact, the Southern Cross is the smallest (in area) of all the constellations. Like the Big Dipper of the northern sky, the Southern Cross indicates the location of the pole 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.
It is thought that Amerigo Vespucci was the first of the European voyagers to see the "Four Stars," as he called them, while on his third voyage in 1501. But actually, Crux was plainly 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. But thanks to precession an oscillating motion of the Earth's axis over the centuries, the Cross ended up getting shifted out of view well to the south.
Immediately to the south and east of the Cross is a pear-shaped, inky spot, about as large as the Cross itself, looking like a great black hole in the midst of the Milky Way. When Sir John Herschel first saw it from the Cape of Good Hope, South Africa in 1835, it is said that he wrote his aunt, Caroline about this "hole in the sky." Indeed, few stars are seen within this hole and it soon became popularly known as the "Coalsack" which initially was thought to be some sort of window into outer space. Today we know that the celebrated Coalsack is really a great cloud of gas and dust that absorbs the light of the stars that must lie beyond it.
Limits of visibility
There are likely a number of readers who have never seen either the Big Dipper or the Southern Cross and might wonder about how far they 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: right after nightfall in late May and early June. To see Crux, one must go at least as far south as latitude 25 degrees north. That means heading to the Florida Keys in the continental United States, where you'll see it just lifting fully above the southern horizon.
So far as seeing 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 soon after sundown. In fact, it's just the opposite effect as opposed to those who live in north temperate latitudes (like New York), whose inhabitants 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!
Interestingly, the Big Dipper and the Southern Cross have also been depicted on a number of flags. The Dipper is depicted on the Alaskan state flag. The Southern Cross can be found on the national flags of Australia, New Zealand, Papua New Guinea, Samoa and Brazil. Interestingly, on the flags of Australia, Brazil, Papua New Guinea and Samoa, Crux is represented with five stars, while on the New Zealand flag only the four brightest stars of the Cross are depicted, the faintest fifth star (Epsilon Crucis) being omitted.
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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.
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