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Springing ahead

March 22, 2011
By Aileen O’Donoghue, aodonoghue@stlawu.edu

After "springing ahead" into Daylight Saving Time, evening darkness comes much later in the Adirondacks. The increase in the hours of daylight is imperceptibly slow near the Winter Solstice, with the days lengthening by only a few seconds. But the rate of change has increased to its maximum at the Vernal Equinox. For more than a month centered on the Equinox, we gain three minutes or more of sunlight each day. Also, if you watch the sun rise or set, the position where it crosses the horizon is about half a degree (equal to its diameter) farther north each day. However much we may enjoy winter and the night sky, we all rejoice in the return of the light!

As the days lengthen into April, the Great Bear, Ursa Major, starts to climb high after its winter walk along the northern horizon. The bears in the Adirondacks will soon follow to emerge from their winter dens. The Dipper forms the tail and haunches of the bear. As shown in the diagram, above the Dipper is Muscida, (MUSS'-ID-A), the muzzle, leading the bear on its endless circuit around the pole. Between this and the Dipper, it's possible to pick out a vague shape of the bear's head and shoulders. Fifteen degrees right of Muscida, about one and a half times the width of your fist at arm's length, are two fairly bright, close stars. These are the bear's front paws. Below and slightly right of them about the same distance, are another two close stars that are one back paw. The second back paw is below these at the same distance and is also marked by close stars. Both back legs connect to the back of the Dipper's bowl and the handle to complete the bear.

As well as forming part of the bear, the Big Dipper acts as a signpost for pathways around the sky. The two stars at the end of the dipper are known as the Pointer Stars because they point the way to Polaris, the North Star. My students are always surprised that Polaris is not as bright as they expect it to be due to its fame. What makes it important, though, is not its appearance, but its location. If you stood at the north pole, Polaris would be directly overhead, marking the north end of Earth's rotation axis to within half a degree. As Earth rotates, all the other celestial objects appear to circle Polaris. A long exposure image of the northern sky will reveal circular star trails around Polaris. The line from the Pointer Stars to Polaris can thus act as the hand of a twenty-four hour clock. Since Earth rotates fifteen degrees per hour, that line will shift fifteen degrees counter-clockwise each hour. Though my students have always been told that Earth rotates, they express amazement at the motion of this line when they observe its position over the course of a few hours. The reason clocks move opposite the stars is that they were based on sundials where the shadow cast by the sun moves clockwise through the day (as seen by an observer in the northern hemisphere facing the sundial and thus away from the sun).

Polaris forms the end of the handle of the Little Dipper that arcs back toward the handle of the Big Dipper. Polaris and the Guard Stars, Kochab (KOE'-CAB, ?, Beta) and Pherkad (FERK-AHD', ?, Gamma) are fairly bright and easy to spot. The middle stars, however, get quite dim. In order of brightness, they are ? (Epsilon), ? (Zeta), ? (Delta), and faint ? (Eta). To see ?, one has to have good eyes and good skies.

Between the Dippers snakes the long constellation of Draco, the Dragon. The star Thuban lies between the Guard Stars and the middle star of the Big Dipper's handle that is actually the two stars, bright Mizar and fainter Alcor. When the Pyramids were being built, Thuban was the pole star, but Earth's precession, the 26,000-year wobble of its axis, has now shifted Polaris to that spot. Draco's head is the Lozenge, an asterism that provides a guide to find the Keystone representing Hercules' hips. Kneeling on one knee, Hercules rises feet-first just north (left) of Corona Borealis, the northern crown, an easily identified backwards "C".

Going back to the Big Dipper, the arc of its handle can be used to "arc to Arcturus" (ARC-TOUR'-RUSS) the second brightest star in the northern sky, after Sirius. Its constellation, Botes (BO-OH'-TEEZ, with o's pronounced as in cooperate), is a herdsman, but I see an ice cream cone with little Muphrid (Moo'-frid) dripping off the end. The arc to Arcturus continues with a spike to Spica (SPY'-KA), the brightest star in Virgo. Just above it (westward) is equally bright Saturn. On April 3, Earth will be directly between the sun and Saturn, putting Saturn at a position called opposition. It will then be at its closest (a mere 800 million miles) and brightest (slightly brighter than Spica). It will also be up all night as it rises at sunset and sets at dawn. You'll still need a telescope to see the rings, though, so look for star parties hosted by members of the Adirondack Public Observatory as the nights get warmer!

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Between Botes and Leo is a wonderful part of the sky to explore with binoculars and telescopes. Coma Berenices, Bernice's Hair, hosts a beautiful cluster of about forty stars covering nearly five degrees of the sky. It's a sibling group of stars known as an open cluster 288 light years away and 400 million years old. This cluster is known as the pond and the three close pairs of stars forming the feet of the bear are known as the "three leaps of the gazelle" who has been startled out of the pond by a twitch of Leo's tail. The Arabic names for the stars are, Alula Australis and Alula Borealis (UH-LOO'-LUH) for the southern and northern first leap nearest the pond, Tainia Australis and Tania Borialis (TAH'-NIH-YUH) for southern and northern second leap and Talitha (TAH'-LITH-UH) for the third leap. Talitha's companion, oddly, has no traditional name. Unknown to those who first named these stars and the pond, far beyond the Coma star cluster is the Coma cluster of 10,000 galaxies. Due to the expansion of the universe, the galaxies are speeding away from us at nearly fifteen million miles per hour! An awesome thing to contemplate as spring creeps slowly northward.

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If you have any questions about astronomy, please visit the Adirondack Public Observatory website at apobservatory.org or email Aileen at aodonoghue@stlawu.edu.

 
 

 

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