Planetary Hours

One fundamental principle of cognition is the scission of the Monad into two parts of symbolic opposites: hot and cold, light and dark, hard and soft, raw and cooked, good and evil. The division of the day into planetary hours is based on this way of thinking: the cycle of the day is split into a dark and a light part. The light part is defined by the length of time between sunrise and sunset and the dark part comprises the hours between sunset and sunrise. And since the length of day and night is only equal at the equinoxes, whereby its opposite, the longest day and shortest night or vice versa occurs at the solstices, William Lilly wrote in his Christian Astrology:

1 Planetary Hour = (Sunset - Sunrise) / 12

“It is very true, some of the Ancients have Winter and Summer, made the day and night to consist of equal hours. I mean every hour to consist of sixty minutes, equally; but Astrologists do not so, but follow this method, viz. according to the motion of the Sun both  Summer and Winter, so do they vary their hours in length or shortness.” One measures the time between sunrise and sunset and divides it into 12 equal parts. These are the planetary day hours. The same may be done with the night hours, measured from sunset to next day’s sunrise to find out the length of each of the planetary night hours.

Watch rulers of days, hours and signs, especially beginnings of Sun
and Moon hours as well as rise, culmination and set of planets.

“You must understand that as there are seven days of the week [...] there are seven Planets [...] We appropriate to each day of the week a several Planet; as to Sunday the Sun, to Monday the Moon, to Tuesday Mars, to Wednesday Mercury, to Thursday Jupiter, to Friday Venus, to Saturday Saturn.” 

This order is known as the “Chaldean Order”, derived from the planets' relative mean speeds which are important in horary astrology (HERE).

Calculation of the planetary hours played a certain role in Renaissance astrology and magic. Astronomical tables published in the late 15th or during the 16th century often included a table of planetary hours with their significations.