Sunday (_dies Solis_) was so named from the Sun ruling its first hour. The following six hours being ruled by the other planets, the Sun again rules the eighth hour, also the fifteenth and the twenty-second; the twenty-third hour is ruled by the next planet in the series, Venus;[42] the twenty-fourth by Mercury, and the first hour of the next day by the Moon, hence this will be Monday (_dies Lunæ_). The Moon ruling the first, eighth, fifteenth, twenty-second hours of Monday, the twenty-third hour will be ruled by Saturn (beginning the series again), the twenty-fourth by Jupiter, and the first hour of the next day by Mars; hence this day will be _dies Martis_ or Tuesday, from the planet Tiw = Mars. And so on with the remaining days, the names of the planet ruling the next day being obtained by passing over the name of the two planets ruling the twenty-third and twenty-fourth hours. Thus beginning with the first planet of the series we get:
Saturn ruling Saturday _dies Saturni_ Sun „ Sunday „ _Solis_ Moon „ Monday „ _Lunæ_ Mars (Tiw) „ Tuesday „ _Martis_ Mercury (Woden) „ Wednesday „ _Mercurii_ Jupiter (Thor) „ Thursday „ _Jovis_ Venus (Frigu) „ Friday „ _Veneris_
In the _Knightes Tale_ (Chaucer) Palamon visits the temple of Venus at her hour, then the temples of Diana (the Moon) and of Mars at their respective hours.
Latin Christianity made only two changes: _d. dominicus_ for Sunday, and _d. sabbati_ for Saturday; but the latter still retains its old name in several countries, thus:
Dimenche Dydd Sul Dilun „ Llyn Dimars „ Mawrth Dimècre „ Mercher Dijòu „ Jau Divèndre „ Givener (Wener) Dissate „ Sadwrn
The unit of time is the mean solar-day, the time between the noons of two successive days, noon being taken as the moment of the passage of the sun over the local meridian, corrected by the equation of time or daily correction required to reduce the varying solar days to a mean of all the solar days. So ‘mean time’ is that of a well-regulated clock dividing the year into mean solar-days of 24 hours; there being 365 days, 5 hours, 48 minutes, 46 seconds, in the astronomical year.
It is probable that everywhere, in primitive times, both day and night would be divided, in southern countries at least, or at the equinoxes, into three watches of fairly equal length: the morning, midday, afternoon; and the evening, midnight and dawn. Each of these would become divided, with the rise of astronomical observations and the use of sun-dials, into fourths, making twelve hours for either day or night; twelve hours corresponding to the twelve months of the year or to the uncial divisions of other measures. The civil day would thus be of twenty-four hours, grouped into watches of four hours or into the eight canonical divisions of the day. In medieval times midday was properly the hour of sexte, the sixth hour from prime, the third from tierce; but in course of time the ninth hour, nones, was shifted from 3 P.M. to midday, which thus became ‘noon.’