Planetary Hours

Divides daylight, from sunrise to sunset, and night, from sunset to the next sunrise, into twelve unequal hours each for any date and place, ruled in the Chaldean order (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon) from the planet of the weekday. Sunrise and sunset use the standard solar altitude of −0.833°. Cassius Dio (Roman History 37.18–19) derives the planetary names of the weekdays from this count of hours.

• Cassius Dio, Roman History 37.18–19, tr. Earnest Cary (Loeb Classical Library, 1914), LacusCurtius
• Jean Meeus, Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998), ch. 15, “Rising, Transit, and Setting”