If the night sky of site A is at 20 mag/arcsec2, the brightness of sky there is equivalent to a celestial object of 20 mag filling up a patch of sky of area 1 arcsec x 1 arcsec.

• 如果甲地方的夜空光度是20等每平方角秒，则可以理解为甲地方夜空的光度，相等于一个20等的天体，填满一角秒乘一角秒大的天空。