What planetary hours are
Planetary hours are one of the oldest ways of timing the day. They divide daylight into twelve "hours" and the night into twelve more, and give each one to a planet: the Sun, the Moon, Mercury, Venus, Mars, Jupiter or Saturn. Astrologers use them to pick a time that suits an activity, starting a letter in an hour of Mercury, say, or a date in an hour of Venus. The system goes back to the Hellenistic world, and it gave us the order of the days of the week.
How they are counted
The day's hours run from sunrise to sunset and the night's from sunset to the next sunrise, each divided into twelve equal parts. So the hours are "unequal": in midsummer a day hour can run well over an hour and a night hour well under, and in winter the reverse. The first hour of the day belongs to the planet that rules the weekday, and each hour after it passes to the next planet in the Chaldean order: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, the Moon, and round again.
The planets of the weekdays
After 24 hours the sequence lands, every time, on the planet three places further on in the Chaldean order, and that planet rules the first hour of the next day. That is where the order of the week comes from: the Sun's day (Sunday), the Moon's (Monday), then Mars, Mercury, Jupiter, Venus and Saturn. English kept the Sun, the Moon and Saturn and swapped in Germanic gods for the rest: Tiw for Mars (Tuesday), Woden for Mercury (Wednesday), Thor for Jupiter (Thursday) and Frigg for Venus (Friday). The Romance languages kept the planets: in French, mardi, mercredi, jeudi and vendredi.
What each planetary hour is for
Tradition matches each planet's hour to the things it rules:
- The Sun: leadership, success, health and meeting people in power.
- The Moon: home, family, travel and anything meant to change or grow.
- Mercury: writing, study, messages, trade and short trips.
- Venus: love, beauty, art, pleasure and friendship.
- Mars: courage, competition, exercise and hard conversations.
- Jupiter: growth, luck, law, teaching and asking for favors.
- Saturn: discipline, endings, property and patient long-term work.
Many astrologers combine the hour with the day, so an hour of Venus on a Friday (Venus's own day) counts double. These are traditional associations to use for reflection and fun, not predictions.
How the times are calculated
The calculator finds sunrise and sunset for your town with Astronomy Engine, counting the moment the top edge of the Sun touches the horizon, with the standard allowance for the bending of light by the air, as almanacs do. The site's tests compare it with the NOAA method used by weather services, for six cities from Reykjavik to Sydney across the year, and the two agree within two minutes. Times are shown in your town's own time zone, daylight saving included. In the far north and south, where the Sun doesn't rise or set for part of the year, there are no planetary hours to count, and the calculator says so.