Planetary hours calculator

Everything starts with sunrise and sunset for the place you chose. Our own astronomy engine finds the moment the upper edge of the Sun touches a flat horizon, allowing 34 arcminutes for the bending of light near the horizon, the convention almanacs use. We checked the results against the US Naval Observatory for cities from Quito to Reykjavik, and they agree to within a minute.

Daylight is then cut into twelve equal day hours and the night into twelve equal night hours. The first day hour belongs to the planet that rules the weekday, and every hour after it passes to the next planet in the Chaldean order. Because every hour starts from the true sunrise and sunset at your coordinates, the hour shown is the one running in your own sky, in midsummer and midwinter alike.

The hours come from a time when the day, however long, was counted as twelve hours from sunrise to sunset. Greek and Roman sundials marked these seasonal hours, and the night was counted the same way. A planetary hour keeps that old rule, so its length depends on the season and on how far you are from the equator.

In London a day hour lasts about 83 minutes at the June solstice and about 39 minutes at the December solstice, and the night hours do the opposite. In Istanbul and New York the swing is smaller, from about 46 to 76 minutes. In Quito, almost on the equator, day and night hours stay close to 60 minutes all year. Around the equinoxes, day and night hours are roughly equal everywhere.

The rulers follow the Chaldean order: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, the Moon. It ranks the seven planets visible to the naked eye from the slowest to the fastest against the stars, which ancient astronomers took to be their order of distance from Earth.

Count the 24 hours of one day through that list and the next sunrise lands three places further on. That is why the week runs Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn. The Roman historian Cassius Dio described this count in the early third century. The names survive in many languages: Sunday, Monday and Saturday in English, and mardi, mercredi, jeudi and vendredi (Mars, Mercury, Jupiter, Venus) in French.

A planetary day starts at sunrise, not at midnight. At 4 a.m. on a Monday you are, in most places and seasons, still in the night hours of Sunday, counted from Sunday's sunrise. The calculator handles this for the hour it shows as running now: before sunrise it looks back to the previous day's night.

The table for a chosen date always runs from that date's sunrise to the following sunrise, so its last night hours fall on the next calendar morning. All times are in the local time of the place, with daylight saving time where it applies.

Inside the Arctic and Antarctic circles there are days when the Sun never sets and weeks when it never rises. With no sunrise or no sunset there is nothing to divide into twelve, so planetary hours cannot be defined, and the calculator tells you so instead of inventing a rule. The planet ruling the day still applies, because it depends only on the weekday.

Some modern astrologers fill the gap with equal 60-minute hours or count from twilight, but the classical sources give no rule for it. Just outside the polar circles the hours already turn extreme: in Reykjavik a day hour at the June solstice lasts about 106 minutes and a night hour about 14.

Planetary hours belong to electional astrology, the practice of choosing a time to begin something. The idea is to match the matter to the planet: Venus hours for love and friendship, Mercury hours for writing and trade, Jupiter hours for requests and money, as in the list above. William Lilly, the 17th-century horary astrologer, also used the ruler of the hour to test whether a question chart was fit to judge.

We present the hours as a traditional timing system and compute them as precisely as the astronomy allows. Whether a Venus hour helps a first date is a matter of practice and belief; the sunrise it starts from is a matter of fact.