A lesson was devoted to explaining the "rotating" of the earth around the sun -- when in 
(A lesson; was; devoted to explaining the "rotating" of the earth around the sun) (A lesson; to explaining; the "rotating" of the earth around the sun -- when in)
A lesson A lesson
was to explaining
devoted to explaining the "rotating" of the earth around the sun
the "rotating" of the earth around the sun -- when in