According to Peter de Abano, “…. that the first
hour of the day, of every Country, and in every season whatsoever,
is to be assigned to the Sun-rising, when he first appeareth
arising in the horizon: and the first hour of the night
is to be the thirteenth hour, from the first hour of the
What does this mean? The first hour occurs from Sunrise
and the first hour of the night occurs 13 hours from Sunrise.
To calculate the actual hours you need to know the time
of the sunrise. From this time the other ours are easily
calculated, using 60 minutes per hour.
Peter de Abano: Heptameron, or Magical Elements.