Byrd co-published the collection Cantiones Sacrae in 1575 with Thomas Tallis, 40 years his senior, on the occasion of their being granted by Queen Elizabeth a joint monopoly on music publishing in England. It contained 17 motets by each composer, serving as a retrospective for Tallis and an announcement of arrival for Byrd, and proving that English music had finally risen up to continental standards. The main technical issue that Byrd dealt with in the works up to and including the Cantiones is, broadly, the problem of finding a viable general model for the imitative motet. But numerous other questions emerged as well, each requiring its own solution. As a musical resumé for a young composer in the shadow of his elder, the Cantiones is a fairly diverse collection, representing all the main strains in Byrd's work up to that time.

One of the methods he pursues is loosely based on earlier so-called chanson-motets. What mainly distinguishes chanson-motets is that they are more intimate in tone, more light and lyrical than motets tended to be. Byrd uses this approach in Cantiones when faced with a poetic text deserving a more sensitive treatment, and labels the works Hymns. Siderum rector is a "Hymn for Holy Women, neither virgins nor martyrs," the text being the final two stanzas from Virginis proles, a hymn used in the Matins for Holy Women. The hymns are considered some of the most forward-looking pieces in the entire Cantiones.

Among the main techniques he uses in them are homophony, half-homophony, and antiphon. The Siderum rector text is in fact sung through twice in its entirety, once by half the choir, then again with all the voices. In order to deepen the listeners' involvement, the highest voice takes the melody during the first statement, and the tenor does during the next, drawing us further into the deep spiritual caress of the music. It is such a charming little work, and outwardly so unambitious it is in danger of being condescended as merely pretty, but with Byrd's fine execution and melodic eloquence, Siderum rector is much more than that.

© Donato Mancini, All Music Guide