dyu-es-
“daylight, bright day”deity, divine, dial, diary, diurnal
Abstract from *dyew- giving Latin diēs/dīvus, English deity, divine, dial, diary, diurnal.
Discussion
The form *dyu-es- is a stem formation meaning "daylight, the bright day," derived from the root *dyew- "sky, heaven, brightness," one of the most ancient and culturally significant roots in the Proto-Indo-European lexicon. Pokorny (IEW 183–187) treats the root at length, recognizing it as the source of both the word for day and the name of the supreme sky deity. Rix (LIV² 108–109) reconstructs the verbal base *dyew- with the sense "to shine, to be bright," from which nominal derivatives for sky, day, and god radiate outward.
Latin diēs "day" continues the root in its temporal sense and has produced a rich set of English derivatives: diary (a daily record), dial (originally a sundial measuring the day), diurnal (pertaining to daylight hours), and meridian (the midday line, from medius and diēs). The word dismal, surprisingly, descends from medieval Latin diēs malī "evil days," unlucky dates on the calendar. Far more momentous is the theonymic branch. Latin Jūpiter contracts an original vocative *dyeu-ph₂tér "sky father," preserving both the root and the kinship term in a single sacred name. Greek Zeus (genitive Diós) is the same formation with regular Greek phonological development.
In the Germanic world, the sky god became Tīwaz, whose name survives in English Tuesday — Tiw's day — a quiet memorial to a deity whose importance faded in Norse tradition but whose name still marks the week. Sanskrit diva "sky, heaven" and Vedic Dyáuṣ Pitā "Sky Father" complete the picture, showing that speakers from the Ganges to the Thames inherited the same luminous word for the bright vault above and the god who ruled it.