- The Summer Triangle is an astronomical asterism in the northern celestial hemisphere. The defining vertices of this imaginary triangle are at Altair, Deneb, and Vega, each of which is the brightest star of its constellation (Aquila, Cygnus, and Lyra, respectively). The greatest declination is +45° and lowest is +9° meaning the three can be seen from all places in the Northern Hemisphere and from the home of most people resident in the Southern Hemisphere. The two stars in Aquila and Cygnus represent the head of an eagle and tail of a swan that looks east inscribed into the triangle and forming the altitude of the triangle. Two small constellations, Sagitta and Vulpecula, lie between Aquila in the south of the triangle and Cygnus and Lyra to the north.