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September 10, 2010

Sept. starline: Venus in the West

By Dr. Steve Kipp

          September means the seventh month but quick counting will verify that it is the ninth month. The explanation is simple: September was the seventh month of the early Roman calendar that began with March.  The same explanation follows for October, November and December, the eight, ninth and tenth months of the same Roman calendar. 

Just after sunset in September a very bright Venus is seen setting in the west.  As Venus sets another bright planet, Jupiter, rises in the east. If you inspect Jupiter with binoculars you might notice in the same binocular field a green star that really is the planet Uranus. A few months ago the summer triangle, a giant isosceles triangle in the sky formed by three bright stars, was rising in the east at sunset. In September, by the time it gets good and dark, the summer triangle is directly overhead. The sharp angle of the triangle points south.  But now another geometric pattern is rising in the east just above Jupiter-the great square of Pegasus. The square, slightly smaller than the triangle,  is resting on its corner, looking like a diamond, rising just ahead of bright Jupiter. The four stars of the square are not as bright as the triangle stars so you may need to wait till later after sunset to see them.  Notice that the sky inside the square is empty of bright stars.

          Once you have found the summer triangle and the great square turn north. Polaris the north star is visible halfway up in the north. If you look northeast you will see the “W” pattern of Cassiopia and to the northwest is the big dipper right-side up.      

September 23 brings the autumnal equinox for us in the Northern Hemisphere. On that date the Earth’s tilt is directed perpendicular to the direction to the sun so the Earth is neither tilted towards nor away from the sun. Consequently the sun illuminates the northern and southern hemispheres equally. Equinox means equal night and we usually say that on the equinox the length of the night equals the length of the day but that is not quite right! If you inspect your Minnesota Weather Guide Calendar or any source of sunrise and sunset times you will see that on September 23 the times of sunrise and sunset are about 7:04 AM and 7:12 PM. CDT respectively. The day length is about 8 minutes longer than the night. This difference in the length of day and night on the equinox is due to the way sunrise and sunset are defined and also due to refraction of the disk of the sun. Notice that that sunrise and sunset times are closest about two days after the equinox on September 25!

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