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Northern Rockies Skies for November

October 30, 2012

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

Our engaging winter constellations rise around 9 p.m. The Milky Way spikes across the sky directly to the zenith (overhead) and settles down on the western horizon.

Slightly to the south of the Milky Way and mostly overhead, the two prominent constellations, Pegasus and Cassiopeia, can be seen. The winged horse, Pegasus, is recognized by its famous Great Square. The queen, Cassiopeia, is recognized by the stretched-out “W” or “M” star pattern.

Rising prominently in the east and a definite sign of the arrival of winter is Taurus the bull. This majestic constellation forms a prominent “V” in the sky and is the location of the nearest cluster of stars, the Hyades. Aldebaran, the bright orange star, forms the eye of the bull. Orion, the hunter, follows Taurus.

Planets for this November: Jupiter rises at 8 p.m. on the eastern horizon in Taurus the bull. About 1-2 hours before sunrise, Venus and Saturn can be seen in Virgo, near the star Spica.

Finally, don’t forget the Leonid meteor shower Nov. 17. It could be a great show since the moon is in a new phase.

Famous Astronomers:  Aristarchus

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From the time of the ancient Greek world (around the fourth century B.C.) through the Middle Ages, astronomy was dominated by Arab contributions. Virtually, the whole civilized world believed that the Earth was the center of the universe, and that the sun revolved around the Earth.

No less than the great Greek scientists Ptolemy and Aristotle championed this Earth-centered cosmology until 1543, when Copernicus published the "Revolutions of the Celestial Spheres." A sun-centered solar system was not widely accepted until Newton's exposition of gravitation and the laws of motion -- only three centuries ago.

The work of one important astronomer who differed with this Earth-centered viewpoint, Aristarchus of Samos (ca. 310-230 B.C.), comes to us via "The Sand Reckoner" by Archimedes. This treatise describes the main features of Aristarchus' universe, including a sun-centered system; the Earth revolving around the sun; and a star sphere also centered on the sun, but at a vastly larger distance than any previous model had envisioned.

Aristarchus proposed that the star sphere lies about one trillion miles from the sun. The nearest star, Proxima Centauri, lies at about six trillion miles, a distance measured only in modern times.

In his only surviving work, Aristarchus estimated the distances of the sun and moon. He determined the sun to be 18-20 times more distant than the moon. He employed correct geometric reasoning, using the size of the moon's shadow during solar eclipses as well as the angular diameters of the sun and moon. Yet, the true ratio of their distances is actually close to 400. The discrepancy probably resulted from Aristarchus' errant measurements of the sun-moon-Earth angle at the half-moon phase.

A lunar crater is named for Aristarchus. It is the brightest of the large lunar craters, even visible to the naked eye. The brightness is due to it being a young formation, which is only about 450 million years old.

To view this month’s sky chart, click here.

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