en_tn/rev/08/12.md

a third of the sun was struck

Causing something bad to happen to the sun is spoken of as striking, or hitting, it. This can be stated with an active verb. AT: "a third of the sun changed" or "God changed a third of the sun" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-activepassive)

a third of them turned dark

Possible meanings are 1) "one third of the time they were dark" or 2) "one third of the sun, one third of the moon, and one third of the stars became dark"

a third of the day and a third of the night had no light

"there was no light during one third of the day and one third of the night" or "they did not shine during one third of the day and one third of the night"