An asterism is a distinctive pattern of stars lying within a constellation–or, in some cases, one made up of stars from two or more adjoining constellations. Undoubtedly, the best-known and most easily recognized of all such grouping is the Big Dipper in the constellation of Ursa Major. But each of the four seasons of the year has its own starry asterism, which are not only fascinating in themselves but also make it easy to find your way around the night sky. The guidepost for Fall is that known as the Great Square of Pegasus, located high in the southeastern sky during October. (Those of the other three seasons will be featured in future installments.)
While some asterisms are small enough to fit within the field of view of Edmund wide-angle binoculars, others occupy huge tracts of the sky and are best seen with the unaided eye. This includes all four of the seasonal ones. The Great Square itself is relatively dim, its corners being marked by stars of only the 2nd- and 3rd- magnitude. And it’s actually more a rectangle than a square, which in its current tipped position in the evening sky looks like a celestial baseball diamond! But you can’t miss it, for it encompasses a great blank space that at first glance is seemingly devoid of stars. But although the sky looks empty here, there are actually many faint stellar lights located within its confines - the number visible to the unaided eye being used by many stargazers as an indicator of how dark and transparent the sky is. Some three dozen stars have been counted under excellent conditions. How many can you see here?
As with all the seasonal asterisms, the Great Square can be used to point your way to other interesting sights. For example, a line drawn from its lower right corner star through its upper left corner one points to the constellation of Andromeda (which, in fact, is joined to the Square by the latter star.) Extending this line in the opposite direction points to Aquarius and beyond it Capricornis. As yet another example, drawing a line from the lower left corner of the Square through the upper right one points to Cygnus.
Many other asterisms lie scattered around the Fall sky, as a look at a star map such as the Edmund Scientific Star & Planet Locator will show. (This wonderful rotating chart can be set to show the sky at any hour of the night on any night of the year.) High in the west is the Summer Triangle while low about the southwestern horizon is the Teapot-both of which were briefly mentioned in last month’s installment on the Milky Way. Two others are the Circlet of Pisces, a ring of dim stars located just under the Great Square itself, and the Dolphin, positioned due west of the Square in the constellation Delphinus. (Can you see it jumping out of the celestial waters? Hint: face south and look halfway up the sky.) Both of these small asterisms are nice sights in binoculars, and many other unsuspected wonders like them await the sky explorer.
Related Products
- Edmund Scientific astronomical binoculars
- Edmund Scientific astronomy category
- Edmund Scientific telescopes
- Ward’s astronomy lab activities
Guest post: This article originally appeared on the Edmund Scientific site, as part of their Sky Talk series. The article was written by James Mullaney, former assistant editor at Sky & Telescope and author of five books on stargazing. Edmund Scientific has graciously agreed to let us post these articles to provide additional resources for Earth Science and Astronomy educators.
