Planetary Hours & Days of the Week

A planetary hour is not a clock hour. It is one-twelfth of the time between sunrise and sunset (a day hour) or one-twelfth of the time between sunset and the next sunrise (a night hour). Because the length of daylight changes with the seasons, planetary hours expand and contract throughout the year. In summer, day hours are longer than 60 minutes; in winter, they are shorter. Only near the equinoxes do planetary hours approximate standard clock hours.

Each hour is ruled by one of the seven visible celestial bodies known to the ancient world: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon. The ruler of each hour follows a fixed sequence called the Chaldean order, which cycles endlessly through all 24 hours of the day. The planet that rules the first hour after sunrise also gives its name to the day of the week — this is why Sunday is the Sun's day and Saturday is Saturn's day.

The system of planetary hours originates with the Babylonian astronomers, who ranked the seven visible planets by their orbital periods: Saturn (slowest), Jupiter, Mars, the Sun, Venus, Mercury, and the Moon (fastest). This ranking — known as the Chaldean order — became the foundation for assigning planets to hours and, by extension, to the days of the week.

The Hellenistic period (roughly 300 BCE to 300 CE) formalized the system. Egyptian decans — 36 star-groups used to track the night sky — were adapted into a 24-hour framework mapped onto the seven planets. Vettius Valens (2nd century CE) used planetary hours in electional work. The system passed through the Persian and Arabic traditions, appearing in the Picatrix (10th–11th century) and Agrippa's 'Three Books of Occult Philosophy' (1531).

William Lilly's 'Christian Astrology' (1647) includes detailed instructions for electional timing using planetary hours. The system has been in continuous use from Babylon to the present day — a span of over 2,500 years, making it one of the longest-lived practical astrological techniques.

Each day of the week is named for the planet that rules its first hour after sunrise. The sequence follows from the Chaldean order applied to a 24-hour cycle: since each day has 24 hours and there are 7 planets, the ruler of the first hour advances by 4 positions in the Chaldean sequence from one day to the next.

Sunday belongs to the Sun, Monday to the Moon, Tuesday to Mars, Wednesday to Mercury, Thursday to Jupiter, Friday to Venus, and Saturday to Saturn. In English, the Norse equivalents of the Roman gods give us Tuesday (Tiw/Mars), Wednesday (Woden/Mercury), Thursday (Thor/Jupiter), and Friday (Freya/Venus). Saturday retains the Latin name directly.

When both the day and the hour are ruled by the same planet, the planet's themes are amplified. A Jupiter hour on Thursday, a Venus hour on Friday, or a Saturn hour on Saturday creates a double emphasis — the strongest possible election for that planet's domains.

The calculation requires local sunrise and sunset times for the date and location in question. The time between sunrise and sunset is divided into 12 equal parts — these are the 12 day hours. The time between sunset and the next sunrise is divided into 12 equal parts — these are the 12 night hours.

The first day hour is always ruled by the planet that rules the day of the week. Subsequent hours follow the Chaldean order: Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon, repeating. Since 24 is not evenly divisible by 7, the ruler of the first hour shifts each day, producing the familiar weekly cycle.

In summer, when days are long, day hours can last 80 minutes or more while night hours shrink to 40 minutes. In winter, the reverse is true. At extreme latitudes near the solstices, the imbalance becomes dramatic. This is why a planetary hour calculator needs your location, not just your date — two people on the same day in different cities will have different hour boundaries.

The most common application is electional astrology — choosing when to begin an important activity so that the planetary ruler supports the intended outcome. Want to sign a contract? Choose a Mercury hour. Planning a date? Venus. Filing a legal motion? Jupiter. Starting an exercise program? Mars.

Planetary hours also support daily planning even without specific elections. Knowing which planet rules each hour gives you a framework for scheduling: batch your writing and emails in Mercury hours, your focused deep work in Saturn hours, your creative sessions in Venus or Sun hours, and your exercise in Mars hours.

The system is most powerful when combined with other timing factors — the Moon's sign and phase, the presence or absence of retrogrades, and your own natal chart. But planetary hours work as a standalone timing tool: simple, concrete, and immediately actionable.