Planetary hours (Chaldean)
Western · Timing
Planetary hours are one of the oldest timing tools in Western and Hellenistic astrology: every day is divided into 24 unequal hours — 12 of daylight and 12 of night — with each hour assigned to one of the seven classical planets in the traditional Chaldean sequence.
What it is
The system of planetary hours (Horae Planetarum) traces its roots to ancient Babylonian and Hellenistic astronomy, documented in sources including Vettius Valens (2nd century CE) and referenced in Ptolemy's Tetrabiblos. It became a foundational tool in medieval and Renaissance electional astrology and remains widely used today.
The Chaldean order of the seven classical planets — from slowest to fastest orbital speed — is: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. This sequence is also the source of the names of the days of the week in many European languages. Each day of the week is named after the planet that rules its first daylight hour.
The daylight hours of any day are divided into 12 equal parts between sunrise and sunset. The nighttime hours are divided into 12 equal parts between sunset and the next sunrise. Because days and nights vary in length, the duration of each planetary hour varies throughout the year and by geographic latitude — only at the equinoxes do the hours equal 60 minutes exactly. The planets then rotate through the Chaldean sequence: the 1st hour of the day is ruled by the day's ruling planet, the 2nd hour by the next in the Chaldean order, and so on continuously through day and night.
How it is calculated
To find a planetary hour: (1) Determine sunrise and sunset times for the date and location. (2) Calculate the length of the daylight period; divide by 12 to get the length of each daylight planetary hour. Do the same for the nighttime period. (3) Identify the ruling planet of the day (Sunday = Sun, Monday = Moon, Tuesday = Mars, Wednesday = Mercury, Thursday = Jupiter, Friday = Venus, Saturday = Saturn). (4) The 1st daylight hour is ruled by the day's planet; subsequent hours follow the Chaldean order, cycling through all seven planets repeatedly.
For example, on a Sunday: 1st daylight hour = Sun, 2nd = Venus, 3rd = Mercury, 4th = Moon, 5th = Saturn, 6th = Jupiter, 7th = Mars, 8th = Sun, and so on. The cycle wraps continuously through day and night.
What it reveals
Planetary hours reveal the most fitting quality of time for different activities, based on the significations of the ruling planet. A Mercury hour is favorable for writing, communication, signing contracts, business negotiations, and educational matters. A Venus hour suits romance, creative projects, social engagements, and beauty-related activities. A Mars hour supports assertive action, physical exercise, and competitive endeavors. A Jupiter hour favors expansion, legal matters, teaching, and financial decisions. A Saturn hour is suitable for long-term planning, solitary work, and structural tasks but not for starting new initiatives.
In traditional electional astrology (choosing auspicious moments for important events), planetary hours are combined with other factors — the day's planetary ruler, the Moon's sign and aspects — to identify windows most aligned with the desired outcome.
Frequently asked questions
Are planetary hours the same across different time zones?
Planetary hours are calculated based on local sunrise and sunset times, so they differ by location. The same clock time in two different cities will correspond to different planetary hours because their sunrise and sunset times differ. Always use the local sunrise and sunset for the location where the activity will take place.
Do planetary hours work in both Vedic and Western astrology?
Planetary hours are primarily a Western and Hellenistic technique. The Vedic tradition has its own related system — Hora — which also assigns each hour of the day to a planet, but uses a slightly different framework rooted in Vedic timing conventions. Both systems use the seven classical planets in the Chaldean order, so they overlap significantly in practice.
What is the best planetary hour for starting a new business?
Jupiter hours are traditionally considered most auspicious for beginning business ventures — Jupiter governs expansion, prosperity, and good fortune. Sun hours are also favorable for matters requiring authority and visibility. Mercury hours suit commercial and contractual activities. Avoid Saturn hours for new beginnings (delays), Mars hours if harmony is essential, and Moon hours for matters requiring stability rather than fluctuation.
Classical sources
- Ptolemy, Tetrabiblos
- William Lilly, Christian Astrology
Related techniques
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