Perhaps the best known constellation to most people is the BIG DIPPER. But the truth is the Big Dipper is not a constellation at all. It is an ASTERISM. Asterisms are unofficial groups of stars recognized mostly by amateurs. Since 1930 the International Astronomical Union has recognized 88 official constellations with professional astronomers around the world in agreement. Asterisms may be made up of several stars from a single constellation, or they may pull stars from several different constellations.
The familiar Big Dipper shape is the 7 brightest stars from the constellation Ursa Major (Big Bear), which has 88 stars. In September the Big Dipper is easily seen low in the northwest sky. Nearly straight overhead we can find VEGA in Lyra the harp, the sky’s 4th brightest star. Looking east (left) of Vega is the bright star DENEB in Cygnus the Swan; and then down and below these two is bright ALTAIR in Aquila the eagle. Connect these three stars with imaginary lines and we form the asterism known as the SUMMER TRIANGLE. As we move into fall the Summer Triangle will gradually appear to shift to the western sky and remain visible almost to Christmas.
The eastern sky has another asterism; the GREAT SQUARE OF PEGASUS. It is a nearly perfect square of 4 bright stars which often reminds me of a baseball diamond. Asterisms may be found in all seasons of the year and include the famous 3 belt stars in the center of the winter constellation ORION, and they are fun to look for.
Our planet show continues in September with Jupiter and Saturn appearing due south all month. Jupiter is 8 degrees in front of Saturn and brighter. On September 25th and 26th watch the 1st Quarter Moon pass just below the two planets. A wealth of detail can be seen by those of us with telescopes; binoculars also give great views. But just seeing them and realizing that we are looking out 400 to 700 million miles into the solar system is amazing in itself.
Mars grows ever closer to us throughout September reaching its closest approach to Earth in 2 years in October. By the end of the month it will be as bright as Jupiter. Best viewing time for Mars this month is after midnight when it is nicely up in the southeastern sky. A bright waning gibbous Moon may be seen very close of Mars on September 5th.
Venus remains unmistakably bright in the eastern sky before dawn. Venus rises around 3 am among the stars of Gemini. A waning crescent Moon will be just left of Venus one hour before sunrise on September 14th. September’s Full Moon comes on September 2nd.
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