Its glyph is a circle with an arrow projecting from the upper right, read as a shield and spear. The same sign was later adopted in biology as the standard mark for the male sex. Mars is one of the seven classical planets of Mesopotamian and Hellenistic astrology. Carl Linnaeus paired it with the Venus sign in his 1751 dissertation Plantae Hybridae, marking the male parent of a hybrid plant, and by his Species Plantarum of 1753 he was using it freely as a general symbol for maleness, the direct origin of its ordinary modern use far outside any astrological context. In alchemy the same glyph denoted iron, the metal of weapons and of the war god the planet was named for, a correspondence stable enough by the sixteenth century that a single sign could mean the planet, the metal or, eventually, a biological sex to readers trained in each system in turn.
The Romance weekday descended from Mars, mardi in French, martedi in Italian, preserves the Latin dies Martis, while English Tuesday instead carries the Germanic war god Tiw, substituted for Mars in the same interpretatio germanica that renamed the other weekdays. The pattern holds across four of the seven days and breaks only for the Sun, the Moon and Saturn, whose Old English names kept closer, more literal translations of their Roman originals rather than swapping in an unrelated native deity.