Planetary hours divide each day into 24 parts and give each part to one of the seven classical planets. The tool shows all 24 for the date and city you choose, with the time each one starts and ends, and marks the hour you are in now. The first hour after sunrise always belongs to the planet the weekday is named after; one old explanation of the names of the week rests on exactly this count.
How it works
The day starts at sunrise, not at midnight. The tool finds sunrise, sunset and the next sunrise at your place, then divides the daylight into twelve equal hours and the night into twelve more. These are unequal hours: in New Delhi a daytime hour in 2026 runs from 52 minutes in late December to 70 minutes in late June, and in London from 39 to 83 minutes.
The first daytime hour goes to the ruler of the weekday: the Sun on Sunday, the Moon on Monday, Mars on Tuesday, Mercury on Wednesday, Jupiter on Thursday, Venus on Friday and Saturn on Saturday. Each hour after it goes to the next planet in a fixed order, known today as the Chaldean order, which runs from Saturn down to the Moon: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, and round again.
We follow the rule as William Lilly sets it out in Christian Astrology (1647), at the end of book 2, with his table of planetary hours (pages 474 to 483 of the first edition): the planet of the day begins to rule at sunrise, the daylight is cut into twelve hours and the night into twelve more, and in his worked example a day hour of 1 hour 4 minutes leaves the night hour short of 60 minutes by the same amount.
The order itself is much older. The Roman historian Cassius Dio gives it in his Roman History (book 37, chapter 19) as one of two explanations he had heard for the names of the days: give the first hour to Saturn and the next hours to Jupiter, Mars, the Sun, Venus, Mercury and the Moon, round and round, and the first hour of the next day falls to the Sun. He calls it the order the Egyptians observe. Twenty-four hours move the sequence on by three places, which is why Saturday is followed by Sunday, Sunday by Monday and so on.
Sunrise and sunset are the standard almanac moments, when the top edge of the Sun meets the horizon, with the usual allowance for the bending of light by the air. In our checks, from Quito to Reykjavik, they agree with the Swiss Ephemeris (set to standard air pressure at 10 °C) to within half a minute. Near the poles, where the Sun skims the horizon, they can differ by a few minutes, because there a small change in the allowance for the air moves sunrise and sunset a long way.
Reading the result
The first line names the ruler of the day and, for today, the planet of the hour you are in. The two tables list the day hours from sunrise to sunset and the night hours from sunset to the next sunrise. If you look before sunrise, the tool tells you that the hour belongs to the night of the previous day, because the planetary day has not yet begun.
Traditional astrologers used the hours to pick a time whose planet suited the task. Modern readers use the hours the same way, as a gentle way of planning the day, not as a rule.
Where traditions differ
The same order of planets, starting at sunrise with the planet of the weekday, is used in Indian astrology, where each part is called a hora. Writers differ on small details, such as the exact moment counted as sunrise, and that shifts the hours a little. Dio himself does not say where the count starts or how long an hour is. This tool uses unequal hours counted from the standard almanac sunrise and sunset, as Lilly does.
What it does not do
It does not judge an hour good or bad for you, and it does not use your birth chart. Near the poles, on days when the Sun does not rise, does not set or does not rise again the next morning, there is no whole day and night to divide, and the tool says so rather than guess.
Frequently asked questions
↳ Why is a planetary hour not 60 minutes long?
Because daylight and night are each divided into twelve equal parts. In summer the day hours are long and the night hours short, and in winter the other way round. Away from the equator they are close to 60 minutes only around the equinoxes.
↳ Why does the place matter?
Sunrise and sunset happen at different times in different places, and so do the hours that start from them. Two cities on the same clock can be many minutes apart.
↳ What is the Chaldean order?
It is the name now used for the old order of the seven classical planets, from Saturn down to the Moon: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. Cassius Dio gives the hours in this order and calls it the order the Egyptians observe. The hours run through it again and again.
↳ How do the planetary hours give the days of the week their names?
After 24 hours the sequence has moved on three planets, so the first hour of each day falls to a new planet. Starting from Saturn's day, the next first hours go to the Sun, the Moon, Mars, Mercury, Jupiter and Venus: the order of the weekdays. Cassius Dio gives this as one of two explanations he had heard (Roman History, book 37, chapter 19).
↳ Is this the same as the hora in Vedic astrology?
It uses the same order of planets and the same start at sunrise. Times can differ a little from an almanac that defines sunrise in another way.