The twelve signs. Asterism (astronomy) In astronomy, an asterism is a pattern of stars recognized on Earth's night sky.
It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth. The mostly simple shapes and few stars make these patterns easy to identify, and thus particularly useful to those learning to familiarize themselves with the night sky. Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 90–c 168). Former constellations. The former constellation Argo Navis Former constellations are constellations that are no longer recognized by the International Astronomical Union for various reasons.
Many of these constellations were recognized by authorities for long periods of time, even centuries in many cases, which means they have historical value and can be found on older star charts. List of constellations.