The Nebulous Mass, and the Eyes

Ptolemy in the Tetrabiblos treats the nebulous mass in the breast of Cancer as of the nature of Mars and the Moon, and the tradition that follows gives Praesepe that nature and reads it in a notably grim key, associating this misty cluster above all with the eyes and eyesight, with weak sight and, in the lurid manner of the older sources, with blindness, together with a general reputation for misfortune and disorder. The association of the faint cloud with clouded vision is old and probably owes something to the difficulty of resolving it with the naked eye. It is one of the very few deep-sky objects, rather than single stars, to hold a fixed place in the astrological catalogue.

The Latin name Praesepe, manger, and the two flanking donkeys, Asellus Borealis and Asellus Australis, form a coherent small tableau, the animals feeding at their manger, that is considerably older than the telescope; Ptolemy already describes the "nebulous mass" and its flanking stars in these terms, so the naked-eye picture the name preserves long predates any understanding of what the haze actually was. The blindness and eye-trouble reading Ptolemy attaches to nebulous objects generally recurs for the Pleiades in Taurus elsewhere in this set of tables, part of a single consistent analogical rule rather than a coincidence of two unrelated traditions.