Perseus constellation from Bayer's Uranometria, 1603. Perseus is a constellation in the northern sky, named after the Greek hero Perseus. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations defined by the International Astronomical Union. Located in the northern celestial hemisphere, it lies nearby several other constellations from the legend of Perseus, including Andromeda. Covering 615 square degrees, it ranks twenty-fourth of the 88 constellations in size. It appears prominently in the northern sky during the Northern Hemisphere's spring. Johann Bayer's star atlas Uranometria Omnium Asterismorum was the first atlas to cover the entire celestial sphere. The Uranometria introduced a new system of star designation which has become known as the Bayer designation. His atlas added 12 new constellations to fill in the far south of the night sky, which was unknown to ancient Greece and Rome.