Lost lore of the Little Dipper: this dim constellation, hardly little, may not even be a celestial bear.
WHILE POLARIS is universally known and important as the North Star, its constellation of Ursa Minor, the Little Bear, is not. A big reason is its remarkable lack of deep-sky objects for small telescopes. But this star group attains its high point in the sky on June evenings, so now's a great time to recount some interesting facts about it.
Mystery ofthe due-north Dog's Tail. Last August I mentioned the great English poet John Milton (1608-74). Milton is back this month with a word that he helped to spread across the English-speaking world: cynosure. Not familiar with it? The word's literal meaning was the constellation Ursa Minor, or (probably later) Polaris itself. Figuratively, it's come to mean "a center of all attention" and "a guide," because this constellation and star are the center of the sky's apparent motion for observers in the Northern Hemisphere. They guide us to true north.
But there's an odd and fascinating twist to its story. Cynosure seems to come from Greek words for "dog's tail." Could the Greeks have once considered Ursa Minor to represent the tail of a dog, rather than the unnaturally longtailed bear as we see it? No one knows for sure.
The not-so-Little Dipper. The stars of the Little Bear also make up the familiar Little Dipper. Before glancing at our map on the facing page, can you guess how long the "Little" Dipper really is? The Big Dipper, in a straight line from handle-end to the lip of its bowl, is about 26[degrees] long. Measured the same way, the Little Dipper is almost 19[degrees] long!
Circumpolar stars cross the north-south meridian of the sky twice every 24 hours: first above and then below the celestial pole. The terms to distinguish these events are "upper culmination" and "lower culmination," respectively. I implied earlier that the Little Dipper star pattern has its upper culmination in the evening in June. That's mostly true, but not for Polaris itself. June nightfalls are when Polaris has its lower culmination!
Polaris has such appeal as the North Star because it stays almost stationary as the Earth turns. It swings around at 3/4[degrees] from the north celestial pole, so on June evenings Polaris is not quite 2[degrees] lower than it is in December.
This isn't the case for Kochab and Pherkad, the common names for Beta (R) and Gamma (y) Ursae Minoris. At 2nd magnitude, Kochab is just as bright as Polaris, while Pherkad is 3rd magnitude. They lie 16[degrees] and 18[degrees], respectively, from the celestial pole. So at 40[degrees] north latitude, where the celestial pole stands 40[degrees] above the northern horizon, Kochab and Pherkad are 56[degrees] and 58[degrees] high at upper culmination but only 24[degrees] and 22[degrees] up at lower culmination.
In southern Florida, at lower culmination, Kochab and Pherkad are a mere 9[degrees] and 7[degrees] above the horizon. That's low enough to be severely dimmed by atmospheric absorption, haze, and light pollution. Fred Schaaf welcomes your comments at fschaaf firstname.lastname@example.org.
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|Title Annotation:||Northern Hemisphere's Sky|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2008|
|Previous Article:||June 2008: sky at a glance.|
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