What Is Midnight Sun or Polar Day?
Midnight sun is when at least a part of the Sun's disk is visible above the horizon 24 hours of the day.
The scientific name for midnight Sun is polar day and the opposite is polar night.
No Sunset or Sunrise
As the upper edge of the solar disc disappears below the horizon, the Sun has set, and there is no longer midnight Sun.
The moment the solar disc again becomes visible above the horizon, it is sunrise.
Where Is There Midnight Sun?
Even though Norway is known as the "Land of the midnight Sun" in the tourist industry, there is midnight Sun in several other countries in the Northern Hemisphere: USA (Alaska), Canada, Russia, Greenland, Finland, and Sweden.
Clouds Obscure the Sun
That the Sun is visible above the horizon does not mean that the Sun is shining. Clouds and fog hide the Sun, day or night, and the midnight Sun is only visible weather permitting.
Starts on the North Pole
In the Northern Hemisphere, the midnight Sun starts on the North Pole as the Sun rises around the vernal (spring) equinox in March. Then there is midnight Sun for about six months. The Sun sets again around the autumnal (fall) equinox in September, entering six months of polar night.
As the Sun rises higher in the sky on the North Pole day by day, the midnight Sun gradually expands south until it reaches just south of the polar circle around the beginning of June.
Refraction Bends the Light
One of the reasons that the midnight Sun doesn't end directly on the polar circle, but can also be seen a little further south, is refraction; The fact that light bends when it hits Earth's atmosphere. Refraction makes the Sun visible above the horizon several minutes before it actually gets there.
Changes after Solstice
24 hours of light is perfectly possible without midnight Sun, because it does not get dark as soon as the Sun sets. Before night falls, there are three stages of twilight - civil, nautical and astronomical. During the summer, many areas south of where the midnight Sun can be seen, have what is known as white nights. Nights which are light, but the Sun's disk isn't visible for the enitre night.
Opposite in the Southern Hemisphere
In the Southern Hemisphere, it is opposite. There are polar nights when the Northern Hemisphere has polar days. The only landmass far enough south in the Southern Hemisphere to have midnight Sun, is Antarctica.