极昼，又称白夜、永昼或午夜太阳，是在地球的两极地区，一日之内，太阳都在地平线以上的现象（即全天24小时都是白天）。

太阳直射点在哪个半球，那个半球的极地附近就会出现极昼现象。

极昼的范围与太阳直射点纬度有关，其极点与边界的纬度差就是太阳直射点的纬度。

春分过后，北极附近就会出现极昼，此后极昼范围越来越大；至夏至日达到最大，边界到达北极圈；夏至日过后，北极附近极昼范围逐渐缩小，至秋分日缩至0；秋分过后，南极附近出现极昼，此后南极附近的极昼范围越来越大；至冬至日达到最大，边界到达南极圈；冬至日过后，南极附近极昼范围逐渐缩小，至春分日缩至0。不过由于太阳折射，夏天时极昼的范围可能会超过北极圈（或南极圈）。

The midnight sun is a natural phenomenon that occurs in the local summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the sun remains visible at the local midnight. Around the summer solstice (approximately 21 June in the north and 22 December in the south) the sun is visible for the full 24 hours, given fair weather. The number of days per year with potential midnight sun increases the farther towards either pole one goes. Although approximately defined by the polar circles, in practice the midnight sun can be seen as much as 55 miles (90 km) outside the polar circle, as described below, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year.