Word History: Days and years are natural divisions of time based on the astronomical relation of Earth and the sun, but weeks and the names for the days of the week have their source in astrology. The practice of dividing the year into seven-day units is based on the ancient astrological notion that the seven celestial bodies (the sun, the moon, Mars, Mercury, Jupiter, Venus, and Saturn) influence what happens on Earth and that each controls the first hour of the day named for it. This system was brought into Hellenistic Egypt from Mesopotamia, where astrology had been practiced for millennia and where seven had always been a propitious number. The ancient Romans did not divide their calendar into weeks; they named all the days of the month in relation to the ides, calends, and nones. In AD 321 Constantine the Great grafted the Hellenistic astrological system onto the Roman calendar, making the first day of the week a day of rest and worship and imposing the following sequence of names on the days: Diēs Sōlis, "Sun's Day"; Diēs Lūnae, "Moon's Day"; Diēs Martis, "Mars's Day"; Diēs Mercuriī, "Mercury's Day"; Diēs Jovis, "Jove's Day" or "Jupiter's Day"; Diēs Veneris, "Venus's Day"; and Diēs Saturnī, "Saturn's Day." This new Roman system was adopted with modifications throughout most of western Europe. In the Germanic languages, such as Old English, the names of four of the Roman gods were converted into those of the corresponding Germanic gods. For example, the Germanic god worshiped as Wōden by the pagan ancestors of the Anglo-Saxons and Ódhinn by the Norse (and usually known as Odin in Modern English) was associated with the god Mercury from the Greco-Roman tradition. Both Odin and Mercury were associated with magic, and both oversaw the transfer of souls to the afterworld. Odin inspired poets and was credited with discovering the runes, while Mercury was said to have invented language and writing. Similar correspondences motivated the identification of other Germanic gods with members of the Greco-Roman pantheon. Therefore in Old English we have the following names (with their Modern English developments): Sunnandæg, Sunday; Mōnandæg, Monday; Tīwesæg, Tuesday (Tiu, like Mars, was a god of war); Wōdnesdæg, Wednesday; Thunresdæg, Thursday (Thunor in Old English or Thor in Old Norse, like Jupiter, was lord of the sky; Old Norse Thōrsdagr influenced the English form); Frīgedæg, Friday (Frigg, like Venus, was the goddess of love); and Sæternesdæg, Saturday.