It’s not every day that you get to see an opera danced. That’s exactly what happened when the Mark Morris Dance Group and Music Ensemble brought the iconic 1989 work “Dido and Aeneas” to Northrop on Wednesday. The opera, written by Henry Purcell, was simultaneously sung from the orchestra pit and danced on stage, with intriguing if somewhat disconnected results. Because it was difficult for the audience to see the singers (who were quite good), it was a bit like watching a dubbed movie, in which the dialogue doesn’t exactly line up with the performer who is “speaking.” Still, the live music and singing made up a rich, essential part of the epic work.