Triangulum is a small northern constellation whose three brightest stars, of third and fourth magnitude, form a nearly-isosceles long and narrow triangle.
Since any three points make up the corners of a triangle it is unsurprising, if somewhat unimaginative, to find a triangle among the constellations. Triangulum was known to the Greeks who called it Deltoton, for its shape resembled a capital delta. Aratus described it as an isosceles triangle, having two equal sides and a shorter third side. Eratosthenes said that it represented the Nile river delta. According to Hyginus, some people also saw it as the island of Sicily, which was originally known as Trinacria on account of its three promontories. Trinacria was the home of Ceres, goddess of agriculture. Triangulum contains M33, a galaxy in our Local Group, visible with binoculars.
A smaller triangle, Triangulum Minus, was introduced in 1687 by the Polish astronomer Johannes Hevelius from three stars next to Triangulum. Triangulum Minus was shown on some maps, such as the one reproduced here, but has since fallen into disuse.