This hymn is used for None on Sundays and Weekdays throughout the year in the Extraordinary form and the Ordinary Form.
O Thou true life of all that live!
Who dost, unmov’d, all motion sway;
Who dost the morn and evening give,
And through its changes guide the day:
Thy light upon our evening pour,—
So may our souls no sunset see;
But death to us an open door
To an eternal morning be.
Father of mercies! hear our cry;
Hear us, O sole-begotten Son!
Who, with the Holy Ghost most high,
Reignest while endless ages run.
Words: Attr. St. Ambrose, 4th C.; tr. Fr. Edward Caswall, 1849.
Tune: “Rerum Deus Tenax Vigor” Gregorian Chant, Mode II, traditional.*
*This tone is used for Sundays throughout the year. There are a number of other musical settings for this hymn.
The original Latin text may be found here.