As darkness gathers on early September evenings, the Big Dipper appears in the northwest, about 30 degrees above the horizon. Composed of the brightest stars of the Great Bear, Ursa Major, an ancient constellation, the Big Dipper is an asterism, a star pattern made up of stars of one or more constellations. The bowl stars, Dubhe, 1.79 magnitude, and Merak, 1.80m, termed pointer stars, are guides to the pivotal, though dimmer, Polaris, 1.98m and 48th in luminosity. Polaris is commonly referred to as the North Star or Pole Star. Always in its place, it is useful to know for orientation to location anywhere. To find it, eyeball the distance between Dubhe and Merak, then count about five lengths out from these pointer stars. You will discover Polaris.
The Great Bear’s tail is also a guide: follow the arc of the handle of the Big Dipper to “arc to Arcturus,” a red giant star with an orange hue. Arcturus, minus 0.06m, is the 4th brightest star visible with the naked eye and brightest in the summer sky. Also known as the “Guardian of the Bear,” Arcturus sets in the west at 11:22pm on the 6th, and earlier each successive night.
Returning to Polaris, we find that the star marks the tip of the handle of a rather dim asterism, the Little Dipper. On clear nights in the absence of light pollution, or with binoculars or a telescope, find a second magnitude star, Kochab, 2.08m, at the bottom left of the Little Dipper’s bowl, and top left, Pherkad, 3.00m. The Lesser or Little Bear, Ursa Minor, contains the Little Dipper within: Polaris at the tip of its tail, the bowl of the dipper outlined in stars. Scan the sky to the left of Polaris.
The image of Ursa Major, the Great Bear, is by Sydney Hall (1788 – 1831). It is Plate 9 in Urania’s Mirror 1825, a set of celestial cards. Notice three bright stars on the tail followed, to the right, by four bright stars that form a rectangle. These seven brightest stars outline the handle and bowl of the Big Dipper, an asterism within the Great Bear constellation. Image courtesy of Wikipedia: Featured Pictures. Top image courtesy of EarthSky.org.