Article of the month December

Planetary hours

There are two elaborate sundials on the front of the Ratsapotheke in Görlitz, Germany. We will discuss one of them.
The antique or unequal hours are emphasized in red.
Because there are planet symbols between these hour lines, they are frequently referred to as planetary hours, in modern sundial literature as well as ancient.

In 1925, Joseph Drecker of Germany, in his book Sundial Theory, observed that this is not correct.
Planetary hours should be based on the ecliptic , whereas the antique hours are based on the diurnal arc.

Drecker, in a footnote, refers to Sacrobosco, around 1230. Sacrobosco writes:

Hora naturalis est spatium temporis in quo medietas signi peroritur.
(A natural hour is the space of time in which half a sign rises.)

Because of the angle between the ecliptic and the equator, a half sign (= 15 degrees) rises above the horizon sometimes faster, and sometimes slower.
The rise of a half sign will therefore vary in length, which means that this time system will have unequal hours.

Because six of the twelve signs are always above the horizon, there are always twelve hours between sunrise and sunset, as is the case with antique hours.
Each day, however, the length of the hours differs.

This is well demonstrated using an astrolabe.

Calculation will do as well, and with a computer this is mere child's play.
The hour line pattern for a horizontal nodus dial for 52 degrees North latitude looks like this:

Below, for clarity’s sake, the hour line pattern is split for lengthening and shortening days.
Notice the large difference in the duration of the hours.

Top: Lengthening days
Bottom: Shortening days