This hymn is used for Sext on Sundays and Weekdays throughout the year in the Extraordinary form and the Ordinary Form of the Roman Breviary. It was also used for Sext in the Sarum Breviary.
Rector potens, verax Deus,
Qui temperas rerum vices,
Splendore mane instruis*
Et ignibus meridiem,
Extingue flammas litium,
Aufer calorem noxium**,
Confer salutem corporum
Veramque pacem cordium.
Præsta, Pater piissime,
Patrique compar Unice,
Cum Spiritu Paraclito
Regnans per omne sæculum. Amen.
Words: Attr. St. Ambrose, 4th C.
Tune: “Rector Potens Verax Deus” Gregorian Chant, Mode II, traditional.***
* This line is rendered as the following in the Extraordinary Form:
“Splendore mane illuminas”
**The Hymnale Secundum Usum Insignis Ac Praeclarae Ecclesiae Sarisburiensis (1850) has this word as “noctium.”
***This tone is used for Sundays throughout the year. There are a number of other musical settings for this hymn.