The Astrolium planetary hours calculator returns the Chaldean planetary ruler of every hour of any day, for any location on Earth, in under 30 seconds. Sunrise and sunset are computed exactly from the Swiss Ephemeris; the Chaldean rotation runs over the resulting 12 day-hours and 12 night-hours. No login, no email, no upload.
This is the free preview. The full predictive timing feature layers planetary hours over the daily transit picture so you can pick electional moments inside the larger timing structure. For related electional and timing techniques, see the profections calculator and the Saturn return calculator. For the $29 per month Pro tier with full electional search, see pricing.
What you get
The output is a table. Twelve rows for the day-hours, twelve for the night-hours. Each row gives the start and end time (local), the planetary ruler, and a one-line read on the kind of action the hour supports. The first day-hour after sunrise is ruled by the day-ruler, the planet the day is named after. The sequence then rotates through the Chaldean order, repeating across the full 24-hour cycle.
The Chaldean order
Seven classical planets, ranked from slowest to fastest apparent motion as seen from Earth:
- Saturn, the outermost classical planet, 29.4571-year orbit
- Jupiter, 11.86-year orbit
- Mars, 1.88-year orbit
- Sun, 1-year apparent revolution
- Venus, 224.7-day orbit
- Mercury, 87.97-day orbit
- Moon, 27.3-day sidereal cycle
The ordering is documented in Ptolemy's Tetrabiblos in the 2nd century and survives intact through medieval Arabic and European astrology. The sequence repeats: after Moon comes Saturn again, looping continuously through the 24 hours.
The Chaldean rotation is also why the days of the week are named the way they are. The planet ruling the first hour of one day determines the planet ruling the first hour of the next. Start with Saturn on Saturday morning. After 24 hours, the rotation lands you back on Sun, which gives Sunday its name. Continue the rotation through the week and you generate the entire seven-day weekday sequence in Western languages.
How long is a planetary hour
Not 60 minutes. The day from sunrise to sunset is divided into 12 equal parts; the night from sunset to sunrise into 12 equal parts. In summer at mid-latitudes, day-hours run roughly 70 minutes and night-hours roughly 50. In winter, the reverse. At high latitudes near the solstices, hours can stretch to 90 minutes or compress to 30.
Astrolium computes sunrise and sunset to the second from the Swiss Ephemeris, then divides each into 12 exact equal-parts. Day-hour 1 begins at sunrise. Night-hour 1 begins at sunset.
Day ruler, night ruler, hour ruler
Three layers of rulership in classical electional work.
The day ruler is the planet of the calendar day: Saturn on Saturday, Sun on Sunday, and so on. The day ruler frames the broadest tone of any electional window inside that day.
The night ruler is a separate convention some traditions use, marking the planet ruling the first night-hour after sunset. The first night-hour of Saturday is not Saturn; the rotation continues across the sunset boundary, so the night ruler of Saturday is whichever planet the Chaldean sequence reaches when night-hour 1 lands.
The hour ruler is the planet of the specific hour you're working with. The hour ruler is the precision lever in electional work: same Saturday, different hour, different action supported.
Use cases for electional work
Pick the moment so the planetary energy supports the action.
Sun hour: visibility, leadership, public statement. Schedule the keynote, the press release, the recorded interview.
Moon hour: emotional ground, beginnings of cycles, anything involving the public or a domestic audience. Open a community thread. Plant in a garden. Start a habit.
Mars hour: initiative, contest, surgery, anything requiring direct cutting action. Begin the negotiation. Schedule the procedure. Send the cease-and-desist.
Mercury hour: communication, contracts, negotiation, code. Send the cold pitch email. Sign a contract. Deploy. Mercury rules Wednesday and the hours that bear its name.
Jupiter hour: expansion, growth, fundraising, anything benefiting from generosity or audience. Make the ask. Sign the term sheet. Launch the book. Pitch the investor.
Venus hour: relating, design, aesthetics, anything where charm and proportion matter. Send the love letter. Present the design review. Schedule the wedding.
Saturn hour: discipline, structure, anything requiring boundary or constraint. Set the contract terms. Schedule the difficult conversation. Begin the work that needs to last.
Cross-reference the chosen hour against the day's transit picture. A Jupiter hour during a Saturn-square-Jupiter transit reads differently from a Jupiter hour during a Jupiter trine. The hour sets the foreground; the transits set the background.
Why this matters for traditional astrology
Planetary hours are the foundational electional unit in medieval and Renaissance Arabic astrology, in Hermetic magic, and in the surviving Chaldean source material. William Lilly's Christian Astrology (1647) uses them throughout horary work. John Frawley's The Real Astrology (2000) treats them as a living technique. The Picatrix uses them to schedule talismanic operations.
Astrolium treats planetary hours as a working technique, not a curiosity. The calculator is built for practitioners who use them in client work: picking surgery times, contract signings, business launches, talismans. The math runs on the same Swiss Ephemeris kernel that drives the rest of the platform; the convention is whole-Chaldean with no modern alterations. For deeper background on this tradition, read the hellenistic astrology guide. For questions you want to time an answer to, see the horary calculator.
How the math runs
Swiss Ephemeris DE431 computes sunrise and sunset to the second for the input location and date. The day arc is divided into 12 equal segments; the night arc into 12 equal segments. The Chaldean rotation starts with the day-ruler at sunrise and proceeds through the 24-hour cycle.
The location field auto-detects timezone, latitude, and longitude from any city worldwide. The optional timezone override lets you specify if you're in a region with non-standard offsets. Polar latitudes during the summer or winter solstice produce degenerate cases. When sunrise doesn't happen, Astrolium flags the convention break and offers the civil-twilight workaround used in classical sources for high-latitude work.
Nothing is stored. Nothing is emailed.
After the calculator
A planetary hour reads inside the larger sky. The same Mars hour falling during a Mars-square-Saturn transit reads differently from the Mars hour during a Mars trine Jupiter.
For the structured way to read planetary hours inside the full transit picture, see the predictive timing feature. For the year-frame timing the hour sits inside, run the profections calculator. For the major returns that mark structural moments above the daily hour level, see the Saturn return calculator.
Cross-link
Pair this with the profections calculator for the year's time-lord and the zodiacal releasing calculator for the chapter-level frame. For the major return calculators that mark structural electional moments, see Saturn return, chiron return, jupiter return, and venus return. For the deeper interpretive context on traditional electional work, see the hellenistic astrology guide.