midnight sun(n = noun.phenomenon) - the sun visible at midnight (inside the Arctic or Antarctic Circles);
is a kind of atmospheric phenomenon

   The sun shining at midnight in the arctic or antarctic summer. [Webster 1913 Suppl.]

burn the midnight oil, mean sun, midnight, mock sun, red sun, sun, sun bathing, sun blocker, sun city, sun dance, sun deck, sun gear, sun god, sun hat, sun helmet, sun king, sun lamp, sun lounge, sun marigold, sun myung moon, sun parlor, sun parlour, sun pitcher, sun plant, sun porch, sun protection factor, sun river, sun rose, sun spurge, sun star, sun tea, sun valley, sun visor, sun worship, sun worshiper, sun yat-sen, sun yixian, under the sun