A Classical Planet

Jupiter is the largest classical planet and traditional ruler of Sagittarius and Pisces, associated with the chief of the gods. Its glyph began life as pure abbreviation: a Greek zeta, the initial letter of Zeus, Jupiter's Greek counterpart, marked with a stroke to show it stood for a word rather than a number, a convention shared by several of the other classical planetary signs. In alchemy the same mark denoted tin, completing a set of seven metal-to-planet pairings that had stabilised across European craft and learned writing by the sixteenth century under the Hermetic principle that the heavens and the earth shared one underlying pattern.

The Romance weekday descended from Jupiter, jeudi in French, giovedi in Italian, preserves the Latin dies Iovis, Jove's day, while English Thursday instead carries the Germanic thunder god Thor, substituted for Jupiter in the same wave of interpretatio germanica that gave the other days their Norse and Anglo-Saxon names in place of the original Roman planetary set. Jupiter's own weekday name is one of the clearer surviving links between the classical planetary week and everyday modern language, since giovedi, jeudi and their Romance-language relatives all preserve the Latin genitive Iovis with only minor phonetic wear across more than fifteen centuries of continuous use.