OK, I suppose Orion and Venus are bright enough to be visible through the perpetual twilight of a northern night. But seeing Orion at 11pm in June is a challenge, it being a winter constellation. I'm not sure Venus ever gets far enough from the Sun to be visible after 11pm either, though I grant you that if the sun never gets much below the horizon it could be.

On the night of the practical there is also a bright moon and lots of light visible from the windows of Hogwarts, something one would have thought the astronomy professor would have taken into account with the scheduling since they'd make the practical harder. Unless this is the object of course.