M33, the great spiral galaxy in Triangulum, is considered by many to be the ultimate test of an autumn night's transparency. Known for its low surface brightness (luminosity per unit area on the sky), many an amateur has spent long hours searching for it to no avail.
But unless the sky is absolutely, perfectly pristine, seeing M33 with the eye alone is an impossible task. In fact, back home, spotting it using binoculars through light-polluted suburban skies is challenging enough. The problem is that M33 is big … too big. Although rated at magnitude 6.3, its light covers an area equal to two Full Moons stacked end to end. That causes M33 to have a very low surface brightness. Many people searching for M33 pass right over it unawares. The trick to seeing it, whether with the naked eye, binoculars — or a telescope for that matter — is to use averted vision and scan the area very slowly.