en_tn/psa/089/035.md

his throne

This refers to David's kingdom. (See: rc://en/ta/man/translate/figs-metonymy).

forever like the moon

"to last forever like the moon" (See: rc://en/ta/man/translate/figs-simile)

the moon, the faithful witness in the sky

(See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-personification)