Word History: Homer's Iliad calls him "Zeus who thunders on high" and Milton's Paradise Lost, "the Thunderer," so it is surprising to learn that the Indo-European ancestor of Zeus was a god of the bright daytime sky. Zeus is a somewhat unusual noun in Greek, having both a stem Zēn- (as in the philosopher Zeno's name) and a stem Di- (earlier Diw-). In the Iliad, prayers to Zeus begin with the vocative form Zeu pater, "o father Zeus." Father Zeus was the head of the Greek pantheon; another ancient Indo-European society, the Romans, called the head of their pantheon Iūpiter or Iuppiter—Jupiter. The -piter part of his name is just a reduced form of pater, "father," and Iū- corresponds to the Zeu in Greek: Iūpiter is therefore precisely equivalent to Zeu pater and could be translated "father Jove." Jove itself is from Latin Iov-, the stem form of Iūpiter, an older version of which in Latin was Diov-, showing that the word once had a d as in Greek Diw-. An exact parallel to Zeus and Jupiter is found in the Sanskrit god addressed as Dyauṣ pitar: pitar is "father," and dyauṣ means "sky." We can equate Greek Zeu pater, Latin Iū-piter, and Sanskrit dyauṣ pitar and reconstruct an Indo-European deity, *Dyēus pəter, who was associated with the sky and addressed as "father." Comparative philology has revealed that the "sky" word refers specifically to the bright daytime sky, as it is derived from the root meaning "to shine." This root also shows up in Latin diēs "day," borrowed into English in words like diurnal. · Closely related to these words is Indo-European *deiwos "god," which shows up, among other places, in the name of the Old English god Tīw in Modern English Tuesday, "Tiw's day." *Deiwos is also the source of Latin dīvus "pertaining to the gods," whence English divine and the Italian operatic diva, and deus, "god," whence deity.