I always find this a beautiful part of the dome where the last judgment is portrayed. I do not quite know what it is, given that it contains a moon and a star it probably the "celestial spheres", carried by an angel.
A viewer informed me: This is an angel who closes the Time. The Time is symbolised by a rolled sheet of paper. Inside this Time (e.g. this sheet of paper) are the Sun, the Moon and many stars. Here we see the final moment of the History. The moment when the Time disapears and only Eternity is present. This is the end of the tridimensional Space and the end of the Time.