is a tiny group of stars surrounded by the constellations Perseus to the north, Andromeda to the north and west, and by the Zodiac constellations Aries and Pisces to the south. Although it is a small constellation with only faint stars, Triangulum was known to the ancient star-gazers and was originally called Deltoton, in reference to the shape of the Greek letter Delta. Egyptian astronomers also thought of these stars as a delta, but in the sense of the delta of the river Nile; and so to them it was named Nili Domus, home of the Nile.

Sicilia is the name given this triangle of stars by Roman philosophers, who thought it resembled the triangle-shaped island of Sicily. The Roman goddess Ceres, sister of Jupiter, became the guardian divinity of Sicily as well as the goddess of agriculture and the harvest. She implored Jupiter to replicate her island in the sky. In later times, Sicily became the site for the renowned Palermo Observatory, founded by Pope Pius VII. Completing the cycle, it was here on the first day of the nineteenth century—January 1, 1801—that Giuseppe Piazzi observed a "star" that was not in his catalog. Observing again the following night, Piazzi found the object to have shifted by several minutes of arc. He named this planetoid Ceres Ferdinandea, after his patron goddess and his king. The second part of this name was abandoned in favor of the tradition of naming the planets for mythological figures. When first seen, Ceres was among the stars of Taurus, by coincidence not far from the constellation of Triangulum.

Chinese astronomers include this star pattern together with some stars of Aries and Andromeda to form their asterism Heaven's Great General, Tsien Ta Tseang.

Of the stars of Triangulum, only one has a name. That is Caput Trianguli, head of the triangle.

The Pinwheel Galaxy in Triangulum, M33, is the principal deep-sky object in the constellation. Also referred to as NGC 672, this is a very large (62' × 39') spiral galaxy seen nearly face-on, yet it is not very bright. A member of the Local Group of galaxies, it is only slightly more distant than M32, the beautiful Andromeda Galaxy. To locate it, find M31, then look opposite Mirach, Beta-Andromedae, about an equal distance. Although it rates a magnitude of 5.5, this light is spread out over so large an area that only under the darkest skies can it be seen without binoculars. With a telescope, one needs a wide-field eyepiece, for the galaxy's fuzzy glow spreads wider than the apparent diameter of the Moon. In actual size, the galaxy is about half the diameter of the Milky Way; about one third the size of Andromeda's M31. It reveals areas of current star formation, with its spiral arms dominated by young, hot blue stars, and with the presence of dust and gas. No supernovae have been observed, likely because none of the stars has evolved to an eventual collapse. If there should be one, it would probably outshine the entire galaxy at about sixth magnitude. Supernovae, stars that explode with exceptional violence, are seen more frequently in other galaxies than our own, because there are millions of galaxies, each with billions of stars. Within M33 is a huge cloud of gas, larger than the nebula in Orion. This is NGC 604, a fuzzball with a spectrum similar to that of the Lagoon Nebula in Sagittarius.

Another constellation shares the triangle name but not the heritage. This is Triangulum Australe in the sky of the Southern Hemisphere. The Southern Triangle is one of the twelve star groups that were formed into new constellations by Johann Bayer in his 1603 publication, Uranometria. Bayer had obtained a description of the sky from the Dutch navigator, Pieter Keyser (Petrus Theodori), who died in 1596. This enabled him to fill in parts of the southern sphere unknown to the ancient astronomers. The other constellations of Bayer were Apis, the Bee (now Musca, the Fly), Avis Indica, the Bird of Paradise (now Apus), Chamaeleon, the Chameleon, Dorado, the Swordfish, Grus, the Crane, Hydrus, the Water Snake—not to be confused with Hydra, the Water Serpent—Indus, the American Indian, Phoenix, Piscis Volans, the Flying Fish, (now simply Volans), and Tucana, the Toucan. None of these constellations can be observed from our Northern California latitudes.

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