Hours that change length
Today an hour is always 60 minutes. For most of history, that was not how people counted time.
In the ancient Mediterranean world, the period from sunrise to sunset was divided into twelve equal parts, and the period from sunset to the next sunrise into another twelve. These were called seasonal or unequal hours. In summer, the daytime hours were long and the night hours short. In winter it was the other way round. Only at the equinoxes, in March and September, were all 24 hours roughly the same length.
This made sense in a world without mechanical clocks. Sundials naturally measure this kind of hour, and life was organised around daylight. Seasonal hours stayed in common use in Europe until mechanical clocks spread in the late Middle Ages.
Planetary hours use this old way of counting. To find them for any day, you need the exact times of sunrise and sunset for your location.
The seven classical planets
Ancient astronomers could see seven bodies moving against the fixed stars. They called them all planets, from a Greek word for "wanderers": the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.
They noticed that these bodies move at different speeds. Saturn takes about 29 years to go round the zodiac. The Moon takes about a month. Arranging them from slowest to fastest gives what is known as the Chaldean order:
- Saturn
- Jupiter
- Mars
- Sun
- Venus
- Mercury
- Moon
"Chaldean" was a Greek and Roman name for Babylonian astronomers and astrologers. The order itself became standard in Hellenistic astrology, which grew up in Egypt and the eastern Mediterranean in the last centuries BCE.
How the hours are assigned
The rule is simple:
- The first hour after sunrise is ruled by the planet of that day.
- Each following hour is ruled by the next planet in the Chaldean order.
- After the Moon, the list starts again with Saturn.
For example, Sunday belongs to the Sun. So the first hour after sunrise on Sunday is a Sun hour. Then come Venus, Mercury, the Moon, Saturn, Jupiter, Mars, and back to the Sun, all the way through the day and night.
How this gives us the days of the week
Here is the clever part. A full day has 24 hours, and the cycle of planets has 7. When you count through 24 hours, you end up three places further along the list. So the first hour of the next day falls to the planet three steps on.
Starting with Sunday (Sun) and jumping three steps each time gives:
- Sun: Sunday
- Moon: Monday
- Mars: Tuesday
- Mercury: Wednesday
- Jupiter: Thursday
- Venus: Friday
- Saturn: Saturday
This is exactly the order of the planetary week. The Roman historian Cassius Dio, writing in the early 3rd century CE, gave this explanation for the order of the weekdays. In Latin languages the link is still clear: in Spanish, Tuesday is martes (Mars) and Friday is viernes (Venus). In English, several days took the names of Norse gods instead: Tiw for Tuesday, Woden for Wednesday, Thor for Thursday and Frigg for Friday. Saturday, Sunday and Monday kept their planets.
What planetary hours were used for
In the traditions that used them, each hour took on the character of its planet. These meanings come from astrological lore, not from science:
- Sun: leadership, visibility, health and vitality
- Moon: home, travel, the public and everyday needs
- Mercury: writing, study, trade and messages
- Venus: love, friendship, art and pleasure
- Mars: courage, competition and hard physical effort
- Jupiter: growth, generosity, law and learning
- Saturn: patience, structure, endings and serious work
Planetary hours appear in Greek and Arabic astrological texts and in medieval and Renaissance magic. A well-known medieval book of magic, translated from Arabic and known in Latin as the Picatrix, used planetary timing extensively. Later handbooks printed tables of hours so that people could choose a time to start a journey, sign an agreement or plant a crop.
A worked example
Imagine a summer Tuesday where the sun rises at 5:00 am and sets at 9:00 pm. Daylight lasts 16 hours, or 960 minutes. Divided by twelve, each daytime hour is 80 minutes long. The night lasts 8 hours (480 minutes), so each night hour is only 40 minutes.
Tuesday belongs to Mars, so the first hour, from 5:00 to 6:20 am, is a Mars hour. Then come the Sun (6:20 to 7:40), Venus (7:40 to 9:00), Mercury, the Moon, Saturn, Jupiter, and Mars again. The twelfth daytime hour ends exactly at sunset. In midwinter at the same place, the pattern flips: short daytime hours and long night hours.
Working them out today
To calculate planetary hours properly, you need:
- Your location (latitude and longitude), because sunrise and sunset depend on it
- The date, because day length changes through the year
- Your time zone, so the results show in your local clock time
The tool finds sunrise, sunset and the next sunrise, splits each stretch into twelve, and assigns planets in order. Near the poles, where the sun may not rise or set for weeks, the system breaks down entirely. That is a reminder that it was designed for the latitudes of the ancient Mediterranean.
Different sources sometimes define sunrise slightly differently (for example, whether the top edge or the centre of the sun counts). This can shift the hours by a minute or two.
What the evidence says
There is no scientific evidence that starting something in a particular planetary hour changes the result. Planetary hours are best understood as a beautiful piece of history that shaped our calendar, and as a symbolic way of bringing intention to your day. Some people enjoy using them as a reminder: a Mercury hour for writing that email, a Venus hour to call a friend.
Try it
Our planetary hours clock calculates every hour for your location and date, from real sunrise and sunset times. For another traditional timing tool, see the void-of-course Moon tracker.