The diagram shows the divisions of the Uranian day, which (in the planet's manifestation as Ooranye) is exactly 30 Earth-hours long, that is to say, one and a quarter Terran days.
Note that the diurnal cycle on this giant planet depends not upon the feeble light of the distant Sun but upon the varying airglow, the illumination given off by the aerial biomass, the throom.
The Uranian day begins and ends in the middle of the darkest period, yyne, when the throom is at its faintest.

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