Twelve hours of day, twelve of night, each its own ruler
Planetary hours aren't clock hours. Think of the day as a pie, not a ruler: sunrise to sunset gets cut into twelve equal slices regardless of the season, and sunset to sunrise gets cut into twelve more — so a "planetary hour" runs closer to 70 minutes on a long summer day and closer to 40 on a short winter one.
How the sequence runs
Each of the twelve day-hours and twelve night-hours is ruled by one of the seven planets, cycling in Chaldean order — the planets sorted by their apparent speed across the sky, slowest to fastest:
Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon → (repeats)
The first hour of any day is always ruled by that day's own planet — the first hour of Saturday belongs to Saturn, the first hour of Sunday to the Sun, and so on — which is where the names of the days themselves come from. From there the cycle just keeps running through all 24 hours, planet after planet, wrapping back to Saturn every seven hours.
Worked example — a Saturday
| Hour | Ruler |
|---|---|
| 1 | Saturn |
| 2 | Jupiter |
| 3 | Mars |
| 4 | Sun |
| 5 | Venus |
| 6 | Mercury |
| 7 | Moon |
Hour 8 starts the cycle over at Saturn, and it keeps running through both the day's twelve hours and the night's twelve — 24 hours, 24 steps through the same seven-planet loop.
A talisman for Jupiter's favour was traditionally begun in Jupiter's hour, ideally also on Jupiter's own day (Thursday) — stacking the planet's rulership of the hour on top of its rulership of the day. Working a planet's magic in another planet's hour wasn't forbidden so much as considered to be fighting the current rather than moving with it.