Planetary hour
A planetary hour is one of twelve unequal divisions of daylight or of night, each ruled by a planet in the Chaldean order, which is why the days of the week carry planetary names.
The hours are unequal because daylight and darkness are: a summer daytime hour is longer than sixty minutes and a winter one shorter. The first hour of the day belongs to the ruler of that weekday, and the sequence runs Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon.
Electional astrology uses the hour as a fine timing layer once the day itself has been chosen.
Frequently asked
Why are the hours unequal?
Because they divide the actual daylight into twelve, so their length changes with the season and the latitude.
How do the weekday names follow from this?
Counting the hours in the Chaldean order across a day lands the first hour of the next day on the next weekday ruler.