Planetary Hours Calculator

Planetary hours are an old way of giving each hour of the day a planetary ruler. The daylight and the night are each divided into twelve parts, and each part takes its character from one of the seven classical planets: the Sun, Moon, Mercury, Venus, Mars, Jupiter or Saturn. Astrologers have used them since antiquity to choose a time for a task and to notice the quality of the moment.

Find the time of sunrise, sunset and the next sunrise for your location. Divide the daylight into twelve equal hours and the night into twelve equal hours. The first hour after sunrise belongs to the planet that rules the weekday, and each later hour moves to the next planet in the Chaldean order: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, then Saturn again.

Each planetary hour is one twelfth of the daylight or one twelfth of the night, so its length follows the season. In summer the day hours are long and the night hours short; in winter the reverse. Only near the equinoxes, or near the equator, do both come close to 60 minutes.

The Chaldean order lists the seven classical planets from the slowest-moving, as seen from Earth, to the fastest: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon. Planetary hours follow this order hour by hour. Because 24 hours step through the list three times and three more places, each new day starts three planets further on, which gives the order of the weekdays.

Sunday belongs to the Sun, Monday to the Moon, Tuesday to Mars, Wednesday to Mercury, Thursday to Jupiter, Friday to Venus and Saturday to Saturn. The day's ruler also rules the first hour after sunrise, and the names of the weekdays in many languages still carry these planets or their Germanic counterparts.

At sunrise. The hours between midnight and dawn still belong to the previous day's night, so at 3 AM on a Friday you are in Thursday's night hours. The calculator follows this rule when it shows the hour running now.

Planetary hours need a sunrise and a sunset. During the midnight sun or the polar night there is no horizon crossing to divide, so the calculator says so and shows no hours for that date.

Small differences come from how sunrise is defined (the top edge of the Sun or its center, with or without atmospheric refraction), from the exact coordinates used for a city, and from rounding. This calculator uses the almanac definition: the Sun's upper edge on a sea-level horizon with standard refraction. Times usually agree with other careful sources within a minute or two.