The hymn Aurora lucis rutilat, eleven verses from which "Light's glittering morn bedecks the sky" and "That Eastertide with joy was bright" has it's earliest known appearance in a Frankish hymnal compiled in the 9th C., know as the Murbach hymns. Ergo, it is Ambrosian only in its 88.88 meter, not its authorship. One known copy of this document survives, and can be viewed at the Boldleian Library archives. A summary of this document can be found in their Junius Manuscripts collection.
David Maurand (2023)
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Aurora lucis rutilat. [Easter.] This hymn is ascribed to St. Ambrose, but was not received among his undoubted works by the Benedictine editors. In rendering the hymn into English some translators have given the text in full, whilst others have taken a part only.
This translation by Dr. Neale, in two parts, was published in the Hymnal Noted, in 1852, and continued in later editions. Pt. i. consists of lines 1-20, and 4 lines, and a doxology not in the original, but in the Sarum Breviary, pt. ii. of lines 21-44, and the closing lines of pt. i. repeated.
In 1861, the Compilers of Hymns Ancient & Modern gave this rendering in that collection with rather extensive alterations, and rearranged in three parts, thus:—
i. Aurora lucis rutilat. "Light's glittering morn bedecks the sky.
ii. Tristes erant Apostoli. "The Apostles' hearts were full of pain."
iii. Claro Paschali gaudio. "That Eastertide with joy was bright."
-- Excerpts from John Julian, Dictionary of Hymnology (1907)