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Hours of daylight and darkness

Commentary
Hours of daylight and darkness
The six columns of numbers before the images of the zodiac concern the timing of each day. The duration in hours (red) and minutes (black) is given in Arabic numerals for:
 
  1. Half the night (dusk to midnight)
  2. The full night (dusk to sunrise)
  3. The full night plus half the day (dusk to noon).
In the day indicated in this picture (April 27), the timings are:
 
  1. Half the night: 4 h, 52 m
  2. Full night: 9 h, 49 m
  3. Full night plus half-day: 16 h, 52 m
These timings would constantly change as the days became longer and shorter within the yearly cycle. This was essential to the timing of the liturgy, which was not measured by our system of modern hours, i.e. two sets of twelve hours starting from midnight and noon, but reckoned from natural cues such as dusk and dawn.