What are planetary hours?

What are planetary hours?

A system from the ancient world. Each day and night is split into twelve hours, and each hour is assigned a ruling planet from the seven of classical astronomy — Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. The first hour after sunrise is ruled by the planet of the day, and each following hour takes the next planet in the Chaldean order, cycling round and round through day and night.

This is where the names of the days of the week come from: counting the Chaldean order through twenty-four hours lands the first hour of successive days on the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn — Sunday, Monday, and so on.

The Chaldean order and the unequal hours

The Chaldean order lists the seven planets by their apparent speed against the stars, slowest to fastest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. The hours are “unequal” because a daytime hour is one twelfth of the time from sunrise to sunset and a night hour one twelfth of sunset to sunrise — so in summer the day hours are long and the night hours short, and the reverse in winter, each only sixty minutes at the equinoxes.

The figure shows the Chaldean order and a day split into twelve unequal day hours and twelve unequal night hours.

What they are used for

Interpretation, labelled as such: the tradition uses planetary hours for electional timing — choosing an hour ruled by a fitting planet to begin an activity, a letter to Mercury’s hour, a courtship to Venus’s. It is a scheme of symbolic correspondence between the hour and the planet, applied to the timing of human choices.

Not a physical influence

The planetary hours are a historical convention for dividing and naming time, not a physical force. No measurable influence of a “planetary hour” on events has been established, and nothing here should be read as one. Its interest is as a piece of the history of astronomy and timekeeping — and the origin of the week — not as a mechanism.