For a supernova that is 1 magnitude brighter than the full moon, I think you could roughly expect the sky brightness to be about 1 magnitude brighter than the full moon value. Outside of the city, the full-moon sky brightness is typically about 18 mag/arcsec, so I think if such a supernova went off the sky brightness would be about 17 mag/arcsec. I believe that's a little brighter than the night sky from the suberbs of most cities, it would be roughly as bright as it is directly overhead when the sun is 10 degrees below the horizon (depending on where you're located, but that's typically like 40 minutes after sunset). This is really just order of magnitude, to calculate in detail what the brightness distribution would look like (i.e. how bright it is in the halo around the object, and how bright 90 degrees from the object and so on) is quite a bit more involved. If there were a supernova at ~-20, I could be wrong, but a similar estimate would suggest that you'd get ~-10.5 mag/square arcsecond for the sky background. That'd be roughly what the brightness of the sky directly overhead a little after sunset looks like (the sky would be roughly that bright near the object and then darker away from the object, so you'll have to use your imagination to figure out what that looks like).<br /><br />I think in either case you can expect the object to cast shadows (even the full moon casts shadows when you're in a dark location). <div class="Discussion_UserSignature"> </div>