September will be an interesting month in the sky. The Big Dipper, one of the most recognizable star patterns, will be well positioned for viewing in the northwestern sky. It is made up of seven stars. Four stars on the right form the bowl, and three stars on the left form a bent handle. The front two stars of the bowl point out from the open part of the bowl to Polaris, also called the North Star, because it is always straight north in the sky. Taken together, this is one of the best sights in astronomy.

The Big Dipper is not a constellation. It is an asterism, or recognizable star pattern, within a constellation. The bowl forms the hindquarters and the handle forms the unnaturally long tail of Ursa Major (the Great Bear). Much dimmer stars form the front parts, head, and legs of the bear. When viewed under a dark sky and with enough imagination, the constellation does resemble the stick figure of a bear, or at least an animal.

Polaris is also part of an asterism. It is the end of the handle of the much smaller and dimmer Little Dipper that also has seven stars. Again, three stars form the handle and four the dipper, but it faces in the opposite direction of the Big Dipper. These stars are the only ones in the constellation Ursa Minor (the Little Bear). The black of the sky forms the rest of the little bear.

A pattern of stars is almost always made from a chance line-of-sight alignment of stars that are different distances from Earth. However, in the case of the Big Dipper, all but the front, upper star of the bowl and the star at the end of the handle are related. They all are about 80 to 85 light-years from Earth and are traveling through space together. They were probably formed from the same star-forming nebula and were part of an open star cluster that is drifting apart. Also, if you look at the middle star of the handle, Mizar, and the sky is dark, you should see a dim star close-by. This is Alcor, and together with Mizar, is a naked-eye binary star. Through a telescope, you can see that Mizar also is a binary star. Actually, there are six stars that are gravitationally bound and orbiting around each other while they move through space with the rest of the group.

Ursa Major and Ursa Minor are circumpolar constellations that continually circle around the North Star, and above the mid-northern latitudes, they never set. However, their locations relative to Polaris gradually change during the year, so they can be used as a stellar calendar. The Big Dipper is at its highest and upside-down during spring evenings, to the left of Polaris during summer, under Polaris and near the northern horizon during fall, and to the right of Polaris during winter.

The only planets in the sky during September evenings will be Venus that will be dropping lower in the western sky, and Saturn that will rise in a dim part of the eastern sky and cross the southern sky during the night.

Observing Highlights

Sept. 1: Brilliant Venus will pass below bright Spica, in Virgo (the Maiden). Binoculars may be needed to see Spica.

Sept. 6: The Moon will lead Mars across the sky after they rise at about 2:00.

Sept. 8: The Moon will rise just before very bright Jupiter at about 4:00.

Sept. 13: The Moon will be to the lower right of Venus and to the lower left of Spica early in the evening. The Moon will be to the left of Venus on the 14th.

Sept. 16: The Moon will be to the lower right of bright Antares and very close to the lowest star of a nearly vertical line of three stars that form the front parts of Scorpius (the Scorpion). The Moon will be to the left of Antares on the 17th.

Sept. 19: Star Party sponsored by Grout Museum & Black Hawk Astronomy Club, Prairie Grove Park, Waterloo, 9-10:30.

Sept. 22: Autumn will arrive in the northern hemisphere with the equinox. On this day, everyone will have 12 hours of daylight and the Sun will rise straight in the east and set straight in the west everywhere except at the poles.

Sept. 26: The Moon will lead Saturn across the sky during the night.