Fresh from their triumphant Iowa City premiere of Arvo Pärt’s “Passio,” the Chamber Singers of Iowa City shift musical gears and conclude their 45th season by returning to their musical roots with a performance of two of Johann Sebastian Bach’s finest works, “Magnificat” and “Ascension Oratorio” on May 15.

Bach’s “Magnificat” is a musical setting of the biblical Canticle of Mary, believed to be Mary’s response to the announcement by the angel Gabriel that she is to become the mother of the savior. It is scored for five vocal parts — two sopranos, alto, tenor and bass — and a Baroque orchestra including trumpets and timpani. It was originally written in 1723 in E-flat major, but later changed by Bach to D-major to accommodate the addition of trumpets. It is the second version that the Chamber Singers will be performing. “Magnificat” is written in 12 movements, and was the first major liturgical composition on a Latin text by Bach.

John Eliot Gardiner, in his book “Bach: Music in the Castle of Heaven,” describes Magnificat as “chorally one of the most technically demanding of all Bach’s works.”  What makes it so?  The notes themselves are not particularly difficult, though certainly challenging. The difficulty for singers is to let Bach’s music dance, to let it sing. Learning the music as quickly as possible is important — getting off the page and into the music. Pulses, the press-and-release action of highlighting syllables to make the text come to the fore, make a great difference in how an audience experiences the music — not only hearing it, but also feeling it.

Bach wrote his “Ascension Oratorio,” “Lobet Gott in seinen Reichen, BWV 11,” in 1735, based on the gospels of Luke and Mark and the Acts of the apostles. It is set in 11 movements and is composed in two parts. The first part describes the ascension of Christ, the second describes his disciples’ reaction to the event. So struck were they that an angel had to nudge them back to the present moment with the question, “Why stand staring?”  This could be interpreted as “You have a great deal of work to do. Get to it!”

The careful listener will notice that Bach used the alto aria again much later in the Agnus Dei of his “Mass in B Minor.”

As I finish this column, it is March 21, Bach’s birthday. The Singers regularly rehearse on Monday evenings; it was a delightful coincidence to be singing his music on his 331st birthday.

The Chamber Singers of Iowa City perform Bach’s “Magnificat” and “Ascension Oratorio,” with Baroque orchestra, at 3 p.m. on May 15 at Iowa City High School’s Opstad Auditorium, 1900 Morningside Drive.

Please visit www.icchambersingers.org to buy tickets and learn more about this upcoming concert.

James Petersen sings with the Chamber Singers of Iowa City.