M33, the Pinwheel Galaxy, is a beautiful large face-on spiral galaxy located in the constellation of Triangulum. Also known as NGC 598, and the Triangulum Galaxy, M33 was first discovered more than 350 years ago by Hodierna, and then re-discovered by comet-hunter Charles Messier in 1764. M33 is part of our local cluster of galaxies, including our own Milky Way galaxy, M31, and the Magellanic Clouds. This cluster is also part of a much larger super-cluster of galaxies called the Virgo cluster. M33 covers an apparent area of sky bigger than the full moon, but it also has a relatively low surface brightness. From a dark-sky site under good conditions, it can be seen by keen-eyed observers without optical aid. It is probably the most distant object that can be seen with the naked eye at a distance of 3 million light years, slightly farther than its much brighter neighbor, M31, the Andromeda Galaxy.