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The night sky in spring

April 16, 2013
By Aileen O’Donoghue (aodonoghue@stlawu.edu)

The waxing crescent moon has been climbing higher in the western sunset sky since last Wednesday. Tonight it nestles between the feet of the twins of Gemini, just above Alhena (All-HAY-nah) that marks the heel of Pollux, the immortal twin. To the right of the moon, bright Jupiter points the way to the Pleiades. Below it, Orion stands with club raised.

By the time it's fully dark (astronomical twilight) at 8:30, the Hunter is already tilting toward the western horizon. Betelgeuse (BAY-tle-juice), whose name roughly translates as "the armpit of the great one," will set only four hours after the sun. With the northern hemisphere of Earth tilting toward the sun, the lengthening days quickly take Orion from view. By the middle of May, Rigel (RYE-gel), the bright star marking his left foot, will set with the sun. Betelgeuse will do this by the beginning of June as the sun passes between Earth and this constellation. It will take until late July for the Hunter to fully emerge in the pre-dawn sky to begin his long march to the evening skies of early 2014, when Jupiter will shine from between the twins of Gemini.

Jupiter has been steadily moving away from the Pleiades since its closest approach to this pretty little star cluster at the beginning of February. Like Orion, it is dropping quickly toward the sunset horizon. On April 1, Venus emerged from its passage behind the sun and has been very slowly rising away from the western horizon at sunset. Mercury will follow Venus into the evening sky on May 12, after which the two inner planets will rise as though welcoming their larger brother. The three planets will form a tight trio (within 5) on May 26 and 27. Though the trio will set an hour and a quarter after the sun, it will be a mere 11 above the horizon about the width of your fist held at arm's length so it may be a challenge to spot from the Adirondacks.

Article Photos

Though we are losing the largest planet to dusk, the second largest, with its spectacular rings is rising in the east to grace our summer skies. Saturn will rise at 8:30 tonight. As shown in Figure 1, it is about half way between Kappa Virginis (in star names, the genitive, possessive, case is used) and Alpha Librae, known as Zubenelgenubi (Zoo-ben-el-gen-NEW-bee). It was closest to Zubenelgenubi in early February when it began its retrograde motion, the loop of backwards motion the outer planets make in our sky as faster Earth passes them in its faster orbit. As shown in Figure 2, it will be closest to Kappa Virginis on July 8 when it will stop moving backwards (westward) and begin its more normal, prograde, motion to the east. It will then pass within 2 of Zubenelgenubi in early November, but the sun will have just passed between us and Saturn, so the conjunction will be hidden by the glare of dawn.

In the meantime, we can enjoy Saturn's slow motion between Virgo and Libra through the warm evenings of summer. On April 28, Saturn will be at opposition, opposite the sun in our sky.

Due to the sun's northern position, it will set after Saturn rises on that day. By the time it's fully dark at 10 minutes before 9, it will be almost 12 above the east-southeastern horizon, as though to challenge Jupiter, shining in the western sky. It will rise as Jupiter slips toward the sun. With Venus and Mercury becoming visible in May, four of the five naked-eye planets will be visible in the evening. Only Mars, which will pass behind the sun on Thursday, will be in the morning sky.

Saturn's great beauty, of course, is in its magnificent rings. They are made primarily of chunks of water ice ranging from microscopic to mountain-sized, mixed with some rock and other ices such as methane and ammonia. Recent observations by the Cassini orbiter indicate that they contain too much water to be the debris of a comet, asteroid or moon that strayed close enough to the planet to be ripped apart by its tides. Instead, it is being argued that they formed at the same time as Saturn and the rest of the solar system. Hence the rings seem to be a relic of the early solar system that continue to enrich our understanding of its formation and evolution, as well as enriching the beauty of our cosmic neighborhood.

To see Saturn and its rings for yourself, you'll need a minimum magnification of 25x. To see them clearly, magnifications greater than 100x are needed. Fortunately, the roll-off roof facility at the Adirondack Observatory site above Little Wolf Pond will be up and running this summer to provide telescopic views of this beautiful planet! Keep an eye on our website at apobservatory.org for observing opportunities.

If you do get out to view Saturn, take time to swing north to the Hercules Globular Star Cluster sitting on Hercules' hip like a belt loop as shown in Figure 1. Also known as M13, the 13th "uninteresting fuzzy blur" on Charles Messier's list of things that could be mistaken for comets, this is a cluster of thousands of stars almost as old as the Milky Way galaxy itself! It's over 22,200 light years away and 145 light years in diameter. Turn on a lamp on one side of the cluster and it will take 145 years for it to become visible on the other side. Send a radio signal from Earth, as was done by Arecibo Observatory in 1974, and it will take 22,200 years for it to get to the cluster. If someone picks up our signal and responds, we'll get their answer in 44,400 years! I don't think I'll wait up.

Farther from Saturn in the sky, but much closer to us than M13, is the Coma star cluster.

It's a lovely collection of about 100 stars to admire on warm spring and summer evenings with even compact binoculars. At only 280 light years away, it's in our galactic neighborhood and a mere 450 million years old, compared with M13's 11.65 billion years! It used to represent the end of the tail of Leo the Lion that now ends at Denebola (Den-EBB-oh-la), but in about 240 BC, it was renamed Coma Berenices by Ptolemy III to recognize the legendary sacrifice of her hair for her husband's safe return from battle by Egypt's Queen Bernice. He did return and, legend has it, was not pleased by his wife's short hair! Husbands and wives can be hard to please. Beyond this column, local astronomers are anxious to share our love of the sky with you.

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Check out the Adirondack Public Observatory website at apobservatory.org for events. Listen for me on North Country Public Radio about once a month during "The Eight O'Clock Hour," or email me with any questions at aodonoghue@stlawu.edu.

 
 

 

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