Planetary Hours Calculator

Planetary hours are one of the oldest astrological timing systems, originating in Babylonian and Hellenistic astrology and passed down through medieval and Renaissance traditions. The system divides each day and each night into 12 equal parts — not 12 equal clock-hours, but 12 equal divisions of the actual light and dark periods of that day. This means a “planetary hour” in summer (when days are long) is longer than 60 minutes, while in winter it may be shorter.

The seven classical planets: The system uses only the seven planets known to ancient astronomers: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon. These seven also give us our days of the week — each day is named after its ruling planet (Saturday for Saturn, Sunday for the Sun, Monday for the Moon, Tuesday for Mars in Latin languages, Wednesday for Mercury, Thursday for Jupiter, Friday for Venus).

The Chaldean order: The planets are arranged in descending order of their ancient perceived speed: Saturn (slowest) → Jupiter → Mars → Sun → Venus → Mercury → Moon (fastest). This is called the Chaldean order. The hours cycle through this sequence starting with the day ruler.

How to calculate:

1. Determine sunrise and sunset times for your location and date.
2. Divide the time from sunrise to sunset into 12 equal parts (day hours).
3. Divide the time from sunset to the next sunrise into 12 equal parts (night hours).
4. Hour 1 begins at sunrise. The planet ruling Hour 1 is the day ruler (Sun on Sunday, Moon on Monday, Mars on Tuesday, Mercury on Wednesday, Jupiter on Thursday, Venus on Friday, Saturn on Saturday).
5. Each subsequent hour advances one step through the Chaldean order.
6. Hours cycle: Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon → Saturn → …

Practical use in electional astrology: Electional astrology is the art of choosing the best time to begin an activity. Practitioners use planetary hours to align actions with favorable planetary energies:

• Sun hour: Start important projects, seek recognition, meet with authority figures
• Moon hour: Begin intuitive work, travel, emotional conversations
• Mars hour: Take action, start physical activity, address conflicts directly
• Mercury hour: Sign contracts, make calls, study, write
• Jupiter hour: Expand, invest, seek education, begin spiritual practice
• Venus hour: Begin relationships, create art, make purchases related to beauty
• Saturn hour: Long-term planning, discipline, research, endings

This system has been used by physicians, merchants, and astrologers for over 2,000 years. It remains one of the most practically applied tools in traditional astrology today.