Planetary Hours and Days: How the System Actually Works

The seven-day week is not a natural phenomenon. There is no astronomical cycle of seven days — no orbital period, no lunar phase, nothing in the sky that runs on sevens. The week is a human artefact. And the mechanism that produced it is exactly the planetary hour system that grimoire magicians have been using ever since.

That connection is the most useful thing to know before you open any planetary hours table. Once you understand it, the entire system becomes transparent: why Saturday belongs to Saturn, why an hour is not sixty minutes, why the Picatrix and the Key of Solomon agree on the structure even when they disagree on almost everything else. The logic holds together completely, and it is old enough to have been explained by a Roman historian writing around 200 CE.

This page works through the mechanics from the beginning, then follows the textual lineage from Hellenistic astronomy into the grimoires, and ends with a full worked example.

The Chaldean order

The starting point is a ranking of the seven classical planets that astronomers from at least the Babylonian period arranged by apparent speed across the sky — specifically by their period of revolution against the fixed stars as observed from Earth.

The sequence runs from slowest to fastest:

Saturn — Jupiter — Mars — Sun — Venus — Mercury — Moon

Saturn completes one circuit of the zodiac in roughly thirty years; the Moon manages hers in twenty-seven days. In between, Jupiter takes twelve years, Mars two, the Sun one year, Venus and Mercury oscillating close to the Sun with their own synodic cycles. The ordering by speed is naked-eye astronomy: any careful observer in the ancient Near East could derive it without instruments.

This sequence became known in the Greek and Roman world as the Chaldean order — Chaldaean being the catch-all classical term for Babylonian astronomical learning. Ptolemy uses it in the Tetrabiblos (c. 150 CE) as a cosmological baseline. It organises not just timing but the whole planetary correspondence system: the metals, the days, the hours, the humors, and — in the grimoire tradition — the type of magical operation appropriate to each planet.

The order is the engine. Everything else follows from it mechanically.

How the week grows out of the hours

Divide each day into twenty-four equal segments. Assign the first hour to a ruling planet, then proceed through the Chaldean sequence — cycling back to Saturn after Moon — assigning each successive hour to the next planet in line, wrapping around endlessly.

The first hour of the day defines whose day it is. Work through the arithmetic and you get the weekday sequence.

Start with Saturday. Hour 1 = Saturn. Proceed: hours 1–7 assign Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon; hours 8–14 repeat; hours 15–21 repeat again; hours 22–24 land on Jupiter, Mars, Sun.

Hour 25 — which is hour 1 of the next day — is therefore the next planet in line. Count it precisely: from Saturn (position 0 in a zero-indexed ring of seven), add 24 and take mod 7: 24 mod 7 = 3. Position 3 in the Chaldean sequence is the Sun. So the day after Saturn’s day is the Sun’s day. Sunday. ✓

Repeat: from Sun (position 3), add 24: 3 + 24 = 27 mod 7 = 6. Position 6 is the Moon. Monday. ✓

Continue: Moon is position 6. 6 + 24 = 30 mod 7 = 2. Position 2 is Mars. Mardi in French. Tuesday (from the Norse Tiw, a Mars-equivalent). ✓

Mars is position 2. 2 + 24 = 26 mod 7 = 5. Position 5 is Mercury. Mercredi. Wednesday. ✓

Mercury is position 5. 5 + 24 = 29 mod 7 = 1. Position 1 is Jupiter. Jeudi. Thursday (from Thor, a Jupiter-equivalent). ✓

Jupiter is position 1. 1 + 24 = 25 mod 7 = 4. Position 4 is Venus. Vendredi. Friday. ✓

Venus is position 4. 4 + 24 = 28 mod 7 = 0. Back to Saturn. Saturday. ✓

The familiar seven-day week is the Chaldean hour-sequence operating on a 24-hour cycle. Each successive day’s ruler sits three steps forward from the previous day’s ruler in the Chaldean ring (because 24 mod 7 = 3).

Cassius Dio documented this derivation in his Historia Romana, confirming “the deep connection between the planetary hours system and the foundational structure of the Western calendar.” Dio’s account in Book 37 (c. 200 CE) is the earliest surviving prose explanation of the mechanism — he was explaining to Roman readers why different peoples counted days by planets, and his arithmetic is structurally identical to what every modern planetary hours guide still describes.

Temporal hours, not clock hours

The system uses temporal hours — also called unequal hours or seasonal hours. These are not sixty-minute blocks. They are fractions of the actual day and night at your location and date.

The rule: divide the time from sunrise to sunset into twelve equal parts. Those are your twelve daytime planetary hours. Divide the time from sunset to the following sunrise into twelve equal parts. Those are your twelve night hours.

Because day length varies with latitude and season, a summer daytime hour in northern Europe might run eighty-five minutes, while the corresponding night hour runs only thirty-five. At the equinox, both equal sixty minutes. The system presupposes observation of the actual sky, not a standardised clock.

Agrippa describes this precisely in De Occulta Philosophia Book II (1531): “almost all Astrologers divide all that space of time from the Sun rising to setting into twelve equal parts, and call them the twelve hours of the day” — then repeat the division for the night hours, distributing each to “every one of the Planets according to the order of their successions, giving always the first hour of the day to the Lord of that day.”

The Key of Solomon makes the same point from the operational side: “as there is inequality in the length of the days” — because the comparative duration of day and night varies with sunrise and sunset — “so also the hours of the planets are unequal.” When the day is fifteen hours long, each planetary hour contains seventy-five minutes.

This matters for practice. A modern app that hands you a fixed-length “planetary hour” with a countdown timer is giving you a reasonable approximation, but it is approximating something the tradition defined astronomically. The original system is tied to the actual horizon.

The twenty-four-hour sequence

Each full 24-hour period contains exactly 24 planetary hours — 12 day and 12 night. The full cycle of all seven planets cycling through all positions within a week repeats every 168 hours (168 = 7 × 24). This means the pattern is self-consistent: if you know the current day ruler, you can derive all hours for the week without a table.

The sequence within any given day always follows the Chaldean order, wrapping endlessly. For a Sunday:

Hour	Planet
1	Sun
2	Venus
3	Mercury
4	Moon
5	Saturn
6	Jupiter
7	Mars
8	Sun
9	Venus
10	Mercury
11	Moon
12	Saturn

Night continues:

Night Hour	Planet
1	Jupiter
2	Mars
3	Sun
4	Venus
5	Mercury
6	Moon
7	Saturn
8	Jupiter
9	Mars
10	Sun
11	Venus
12	Mercury

Hour 1 of the following day (Monday) = Moon. Correct: the sequence picks up exactly where it left off, and the next day’s ruler is three steps forward in the Chaldean ring. ✓

Worked example: Sunday, 21 June, London

Step 1. Establish sunrise and sunset. London on the summer solstice: sunrise approximately 04:43 BST, sunset approximately 21:21 BST.

Step 2. Calculate day length. 21:21 minus 04:43 = 16 hours 38 minutes = 998 minutes.

Step 3. Calculate daytime planetary hour length. 998 ÷ 12 = 83.2 minutes per day hour (approximately 1 hour 23 minutes).

Step 4. Calculate night length. 1440 − 998 = 442 minutes.

Step 5. Calculate night planetary hour length. 442 ÷ 12 = 36.8 minutes per night hour (approximately 37 minutes).

Step 6. Identify the day ruler. Sunday = Sun. Hour 1 of the day starts at sunrise.

Step 7. Build the day schedule.

Hour	Planet	Starts (approx BST)
1	Sun	04:43
2	Venus	06:06
3	Mercury	07:29
4	Moon	08:52
5	Saturn	10:15
6	Jupiter	11:38
7	Mars	13:01
8	Sun	14:24
9	Venus	15:47
10	Mercury	17:10
11	Moon	18:33
12	Saturn	19:56

Sunset at 21:21 opens the night sequence at 36.8 minutes per hour:

Night Hour	Planet	Starts (approx BST)
1	Jupiter	21:21
2	Mars	21:58
3	Sun	22:35
4	Venus	23:12
5	Mercury	23:49
6	Moon	00:26
7	Saturn	01:03
8	Jupiter	01:40
9	Mars	02:17
10	Sun	02:54
11	Venus	03:31
12	Mercury	04:08

The following sunrise at roughly 04:43 opens Monday’s first hour: Moon. ✓

Notice what this table makes legible. If you want to work under Venus on this particular Sunday, your daytime windows are hour 2 (06:06) and hour 9 (15:47); the night offers hour 4 (23:12). If you want Saturn — for banishing, constraint, or works requiring long duration — daytime options are hour 5 (10:15) and hour 12 (19:56). The twelfth hour of Saturn at near-sunset has a particular symmetry: the dying light, the threshold hour, the planet of endings closing the day.

The textual lineage

Hellenistic astronomy

The Chaldean order was established cosmological doctrine by at least the second century BCE in Babylonian and Greek practice. By Ptolemy’s time (c. 100–170 CE), it was assumed as shared technical knowledge. The Tetrabiblos uses the sequence throughout as the foundation for planetary rulerships, triplicity, sect, and exaltation. Ptolemy does not argue for the order; he inherits it.

The seven-day planetary week spread across the Roman Empire during the first and second centuries CE. Dio’s explanation implies it was already widely observed by his time (c. 200 CE). The week of seven days in the sequence we still use was a social fact in Rome before Constantine formalised it in a calendar decree of 321 CE.

Picatrix

The Picatrix — an Arabic compilation of astrological magic translated into Latin in the thirteenth century — contains detailed instructions for timing magical operations by planetary hour and day throughout its text. The Arabic original (Ghayat al-Hakim, “The Goal of the Sage”) dates to around the eleventh century; the Latin translation was made around 1256 for the court of Alfonso X of Castile. The Picatrix is not primarily a timing manual — it is a Neoplatonic synthesis of cosmology, natural magic, and talismanic practice — but many of the planetary talisman recipes from the Picatrix require that the talisman be made in the planetary hour of the appropriate planet. The text transmits the doctrine as an established technical requirement.

The Key of Solomon

The Key of Solomon is “the only ordinary Magical Ritual which regulates the operations of Magical Art in accordance with a formal attribution of certain hours in the day and night to the rule and influence of certain planets.” The Clavicula Salomonis manuscripts date from at least the fourteenth and fifteenth centuries. Almost every operation in the text specifies both the day and the hour; the explanation of unequal hours appears within the text itself. The Key’s operational specificity is its distinguishing feature: hours of Saturn and Mars are assigned to works of constraint and discord; hours of Jupiter to honour and legal matters; hours of the Sun to authority and visibility; hours of Venus to love and alliance; hours of Mercury to commerce and communication; hours of the Moon to travel and rapidly changing situations.

Agrippa

Agrippa’s Three Books of Occult Philosophy (first printed Cologne, 1531) is acknowledged as a significant contribution to the Renaissance discussion of magic’s relationship to celestial philosophy. Book II is the fullest early modern theoretical treatment of the planetary hours. Where the grimoires give operational instructions, Agrippa gives the rationale: the hour belongs to the planet because the planet’s influence is strongest at that time. This is the Neoplatonic doctrine of celestial sympathy applied to scheduling. The hour is not a convention; it is a claim about the actual quality of time.

Book II opens with Agrippa’s image of the magus as a go-between uniting the heavens and earth; he details how celestial bodies “emit divine influences that shape the material world,” and argues that by understanding the laws governing how celestial influences descend, the practitioner can “collaborate with nature” to produce desired effects. The planetary hours are one of the most direct mechanisms for that collaboration.

What the system is actually for

The planetary hours scheme is first and foremost a timing system for magical operations. The grimoires use it as a technical requirement: perform the operation in the wrong hour, and the text implies it will fail. The Key of Solomon encodes this as procedural law. Agrippa encodes it as celestial mechanics.

For contemporary practice, the operational schema holds whether or not you accept the Neoplatonic metaphysics. The system is internally consistent, historically documented, and practically specific. That combination is rarer than it should be in the correspondence tradition.

To time a working: choose the day whose ruler aligns with your goal, then work in the hour of either the same planet (double emphasis — Saturn hour on Saturday for maximum Saturnine weight) or a supporting planet (a Venus hour on a Jupiter day for a working that combines attraction with expansion). The Picatrix and Agrippa both hold that the hour is more important than the day when precision is required; planetary hours are more focused than the planetary day alone.

Honest limits and common errors

Latitude matters. The temporal hour calculation depends on your actual sunrise and sunset. An app computing planetary hours for the wrong location is giving you someone else’s schedule. At extreme latitudes, summer day hours can run past ninety minutes while winter day hours shrink below thirty. The original system was calibrated to Mediterranean latitudes; in the far north, the unequal hours become structurally strange and the tradition offers no explicit guidance.

Apps are approximations. Most planetary hours apps use a simplified calculation. They are useful for planning. For operations where the textual tradition specifies precise timing, the manual calculation — sunrise, sunset, divide by twelve — matches the source texts more faithfully.

Day ruler vs. hour ruler. The grimoires treat these as distinct. Working in the Saturn hour on a Saturday is a double emphasis. Working in the Saturn hour on a Sunday is Saturn operating within the Sun’s day — a different quality of work. Modern practitioners often conflate them. The distinction is explicit in the Key of Solomon and in Agrippa.

The hours begin at sunrise, not midnight. Agrippa states this. The grimoires assume it. Hour one is the first hour of daylight. Calculating from any other starting point produces wrong assignments for the entire day. This is where most beginner errors concentrate.

Reading on

The planetary hours are among the most structurally sound pieces of the Western magical tradition — consistent across independent textual lineages from Hellenistic astronomy through medieval Arabic magic through Renaissance Europe, and unchanged in their core mechanics from Cassius Dio’s description to the present. The calculation method is the same. The Chaldean sequence is the same. The operational logic is the same.

For the domains of one of the most practically significant planets in the system — constraint, time, long-term workings, the metals of lead — see the Saturn correspondences page on this site. The Jupiter and Moon pages are forthcoming.