Planetary hours
Hellenistic, on the 'Chaldean' order of the planets — the system that named the days of the week
Daylight and night are each divided into twelve unequal hours, ruled in turn by the planets in descending order of apparent speed (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon). This is where the weekday names come from: run the 24-hour cycle and the ruler of each day's first hour steps through Saturn's-day, Sun's-day, Moon's-day and so on — the order Cassius Dio records for the Roman week. Neugebauer noted that the 'Chaldean' label is really a Greek-era arrangement, not a Babylonian inheritance.
What it measures
Sunrise to sunset divided into twelve 'day hours' and sunset to sunrise into twelve 'night hours', computed from the real rise and set at your coordinates — so an hour is rarely 60 minutes. The first hour of the day is ruled by that weekday's planet, then the sequence runs in Chaldean order.
Still open
Because sunrise is reckoned from the Sun's upper limb and the atmosphere lifts the disc into view early, daylight never exactly equals night — so a planetary hour is never quite 60 minutes even at the equator or an equinox.
Sources
- Cassius Dio, Roman History, Book 37 (on the week)
- Otto Neugebauer, The Exact Sciences in Antiquity (1957)