“This hymn is based upon the hymn O Lux Beata Trinitas, which is ascribed to St. Ambrose (340-397). This version is the result of Pope Urban VIII’s efforts to cast the hymns of the Roman Breviary into classical models. In the old Roman Breviary it is the Vespers hymn for the ferial office on Saturdays and Trinity Sunday.”
| IAM sol recedit igneus: Tu lux perennis Unitas, nostris, beata Trinitas, infunde lumen cordibus. |
AS fades the glowing orb of day, to Thee, great source of light, we pray; blest Three in One, to every heart Thy beams of live and love impart. |
| Te mane laudum carmine, Te deprecamur vespere; digneris ut te supplices laudemus inter caelites. |
At early dawn, at close of day, to Thee our vows we humbly pay; may we, mid joys that never end, with Thy bright Saints in homage bend. |
| Patri, simulque Filio, tibique sancte Spiritus, sicut fuit, sit iugiter saeclum per omne gloria. |
To God the Father, and the Son, and Holy Spirit, Three in One, be endless glory, as before the world began, so evermore. |
https://www.preces-latinae.org/thesaurus/Hymni/IamSol.html
This and much more, via Laura Wood