 the triangulum galaxy is a spiral galaxy approximately 3 million light-years (ly) from earth in the constellation triangulum. it is catalogued as messier 33 or ngc 598, and is sometimes informally referred to as the pinwheel galaxy, a nickname it shares with messier 101. the triangulum galaxy is the third-largest member of the local group of galaxies, which includes the milky way, the andromeda galaxy and about 44 other smaller galaxies. it is one of the most distant permanent objects that can be viewed with the naked eye. ==etymology== the triangulum galaxy is sometimes informally referred to as the "pinwheel galaxy" by some amateur astronomy references and in some public outreach websites. however, the simbad astronomical database, a professional astronomy database that contains formal designations for astronomical objects, indicates that the name pinwheel galaxy is used to refer to messier 101, and several other amateur astronomy resources and other public outreach websites also identify messier 101 by that name. ==visibility== under exceptionally good viewing conditions with no light pollution, the triangulum galaxy can be seen with the naked eye. it is one of the most distant permanent objects that can be viewed without the aid of a telescope. being a diffuse object, its visibility is strongly affected by small amounts of light pollution. it ranges from easily visible by direct vision in dark skies to a difficult averted vision object in rural or suburban skies. for this reason, triangulum is one of the critical sky marks of the bortle dark-sky scale. ==observation history== the triangulum galaxy was probably discovered by the italian astronomer giovanni battista hodierna before 1654. in his work de systemate orbis cometici; deque admirandis coeli caracteribus ("about the systematics of the cometary orbit, and about the admirable objects of the sky"), he listed it as a cloud-like nebulosity or obscuration and gave the cryptic description, "near the triangle 
Astronomical_objects_discovered_in_1654 Local_Group Messier_objects NGC_objects Objects_within_10_Mly_of_Earth PGC_objects Spiral_galaxies Triangulum_(constellation) Triangulum_Galaxy Triangulum_Subgroup UGC_objects Unbarred_spiral_galaxies