The amount of airglow and zodiacal light is quite varied (depending, amongst other things on sunspot activity and the Solar cycle) but given optimal conditions the darkest possible sky has a brightness of about 22 magnitude/square arc second. If a full moon is present, the sky brightness increases to about 18 magnitude/sq. arc second depending on local atmospheric transparency, 40 times brighter than the darkest sky. In densely populated areas a sky brightness of 17 magnitude/sq. arc second is not uncommon, or as much as 100 times brighter than is natural.