An old way of dividing the day
Planetary hours come from Hellenistic and later medieval astrology. Each hour of the day and night is assigned to one of the seven classical planets: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. Astrologers who use them pick an hour whose planet matches the kind of task at hand.
In that tradition, a Venus hour is linked with art, beauty and social ease, a Mercury hour with writing and messages, a Mars hour with effort and competition, and a Saturn hour with patient, serious work.
Why the hours are not 60 minutes
These are unequal hours. We take the time from sunrise to sunset and divide it into twelve parts, then do the same with the time from sunset to the next sunrise. In midsummer the daytime hours run well over 60 minutes and the night hours run short; in midwinter the reverse is true.
Only near the equinoxes in March and September do day and night hours come close to an even hour each. The farther you live from the equator, the bigger the seasonal difference.
The Chaldean order and the names of the days
The planets follow the Chaldean order, sorted from slowest to fastest as seen from Earth: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. After the Moon the sequence starts again with Saturn, around and around.
The first hour after sunrise belongs to the planet that rules that day: Sunday the Sun, Monday the Moon, Tuesday Mars, Wednesday Mercury, Thursday Jupiter, Friday Venus and Saturday Saturn. Count 24 hours forward in the Chaldean order and you land on the next day's ruler, which is how the week got its order in the first place.
How our tool finds your hours
We calculate sunrise and sunset for your exact location with Astronomy Engine, using the moment the top edge of the Sun meets the horizon. The day and night arcs are each split into twelve and labeled with their planets.
Your location matters. Even within one time zone, a city on the east side sees sunrise minutes earlier than one on the west side, and every planetary hour shifts with it.
Planetary hours far north
Close to the poles there are days with no sunset in summer and no sunrise in winter. On those days traditional planetary hours cannot be defined, and the tool will tell you so instead of showing a made-up schedule.
For other timing tools, see the Moon sign today and the Mercury retrograde dates.