The Almanac · Calendars & tools
The Almanac
Planetary hours
Calculate the twelve day and twelve night planetary hours for any date and location, reckoned from local sunrise and sunset in the Chaldean order.
What are planetary hours?
Planetary hours divide the daylight from sunrise to sunset into twelve equal 'hours', and the night from sunset to the next sunrise into twelve more, so their length changes with the seasons. Each hour is ruled by one of the seven classical planets in the 'Chaldean order' — Saturn, Jupiter, Mars, the Sun, Venus, Mercury, the Moon — beginning with the planet that rules the day: the Sun on Sunday, the Moon on Monday, Mars on Tuesday, Mercury on Wednesday, Jupiter on Thursday, Venus on Friday and Saturn on Saturday. The scheme explains the order of our weekday names.
The system is ancient, attested in Roman times, and it became a central timing rule of learned magic. The Key of Solomon and the Heptameron instruct the operator to work in the day and hour of the planet suited to the purpose, and Renaissance and later astrologers used the hours for choosing times. The hours here are computed from the sunrise and sunset at your location.