Midnight sun and polar night

Imagine it is midnight. You look out of the window. The Sun is still in the sky!

This happens in summer in places far, far north and far south. It is called the midnight sun.

In most places, the Sun rises in the morning and sets in the evening every single day. But far to the north and far to the south, the year works differently. In summer there are days when the Sun stays above the horizon for all 24 hours: this is the midnight sun.

In winter there is the opposite: days when the Sun never comes up at all. This is called the polar night. In Norwegian it is called mørketid, the dark time, and the midnight sun is called midnattssol.

Store norske leksikon, the Norwegian encyclopedia written by specialists, defines midnight sun (midnattssol) as the Sun being above the horizon for the whole day, and mørketid as the time of year when the Sun's center does not rise above the local horizon for a whole day. It calls mørketid the opposite of midnight sun.

North of the Arctic Circle, midnight sun can occur between the March and September equinoxes; south of the Antarctic Circle, between the September and March equinoxes. Exactly how many days a place gets depends on its latitude, the height of the horizon around it and the air, as later sections explain.

Earth leans to one side as it goes around the Sun.

In summer, the top of Earth leans toward the Sun. The Sun shines on it all the time, even as Earth spins. In winter, it leans away and stays in the dark.

Earth spins once a day, which makes day and night. But Earth's axis is tilted, so around June the North Pole leans toward the Sun. Near the top of the world, places stay in sunlight even as Earth turns all the way around. Around December, the North Pole leans away, and those places stay in Earth's shadow.

The line that marks where this begins is the Arctic Circle, at about 66 degrees north. On the June solstice, around June 21, the Sun does not set there; on the December solstice, around December 21, it does not rise. The Antarctic Circle does the same in the south, at the opposite times of year.

The polar circles sit at about 66 degrees 33 minutes north and south, 90 degrees minus Earth's tilt. The U.S. National Snow and Ice Data Center (NSIDC) explains that the Arctic Circle marks the latitude above which the Sun does not set on the summer solstice, on or about June 21, and does not rise on the winter solstice, on or about December 21.

When a hemisphere tilts toward the Sun, the circle of latitude around its pole lies entirely on the sunlit side of Earth, so rotation never carries it into night. Half a year later, the same circle lies entirely in Earth's shadow. NSIDC adds that above the Arctic Circle there is little or no solar energy in winter, and what does arrive comes in at a low angle, while in summer the Sun shines for many hours or around the clock.

At the Arctic Circle, the Sun stays up for about one day.

Closer to the North Pole, it stays up for longer. At the North Pole, it stays up for half the year!

Right on the Arctic Circle, the midnight sun lasts only about a day. Travel north and it lasts longer. In Tromsø, a city in northern Norway at about 70 degrees north, the local tourist board says the midnight sun usually lasts from around 20 May to 22 July.

At the North Pole itself, NSIDC explains, the Sun rises once a year and sets once a year: six months of daylight, then six months of night. The South Pole is the same, at opposite times. NOAA staff at the South Pole saw the Sun set on March 20, 2024, and then waited six months for it to rise again.

Store norske leksikon notes that the number of days with midnight sun increases from the polar circle toward the pole. NSIDC describes the extremes: at the North Pole, six months of continuous daylight and six of continuous night; at latitudes between the Arctic Circle and the pole, an intermediate number of days.

Visit Tromsø gives the midnight sun in Tromsø, at 69.6 degrees north, as usually lasting from around 20 May to 22 July, and Store norske leksikon describes the Sun returning there after 49 days of mørketid. At the South Pole, NOAA reported that station staff see just one sunrise and one sunset a year; the sunset of March 20, 2024, began six months of darkness for its South Pole Observatory.

In the polar night, the Sun does not come up.

But the sky is not always black. At lunchtime, the sky can still glow softly. That soft light is called twilight.

During the polar night, the Sun stays below the horizon, but in many places it is only just below. Its light still lights up the sky, a glow called twilight. Store norske leksikon explains that until the Sun is 6 degrees below the horizon, you can still read by a window facing west on a clear evening.

The air also plays a trick. It bends sunlight, so we can see the Sun when it is really just under the horizon. That moves the edge of the midnight sun about 90 kilometers south of the Arctic Circle.

Twilight is light from a Sun below the horizon, scattered by the atmosphere. Store norske leksikon divides it into civil twilight, ending when the Sun's center is 6 degrees below the horizon; nautical, ending at 12 degrees; and astronomical, ending at 18 degrees, when night begins. Near the polar circles, the midwinter Sun dips only a few degrees below the horizon, so midday still brings hours of twilight.

Refraction, the bending of sunlight in the atmosphere, raises the Sun's apparent position. As a result, Store norske leksikon says, the midnight sun limit lies about 90 km south of the Arctic Circle, and at sea level with a clear southern horizon the southern limit of mørketid at the winter solstice is 67 degrees 8 minutes north, not the circle's 66 degrees 33 minutes.

People live in places with midnight sun.

In summer, they can go for a walk at midnight in the sunshine. In winter, the city lights shine on the snow.

Tromsø is one of the places where people live with both. In summer, Visit Tromsø describes hiking after dinner and kayaking at midnight under the Sun. In winter, the people of Tromsø go for weeks without seeing the Sun rise.

At the South Pole, the dark is harder. NOAA explains that in winter it is so cold and dark that aircraft systems don't work, so for nine months, six of them in darkness, no flights go in or out of the research station. That is why NOAA's staff there stay for a whole year at a time.

Life adapts to the light. Visit Tromsø promotes the midnight sun season, around 20 May to 22 July, for evening hikes and midnight kayaking, and warns that weather can change quickly, so walkers should check the forecast before heading into the mountains or out to sea.

At the South Pole, NOAA's South Pole Observatory sits at 2,837 meters above sea level on the Antarctic plateau, part of the National Science Foundation's Amundsen-Scott South Pole Station. Staff serve one-year tours because, NOAA explains, for nine months of the year (six of them dark) aircraft cannot fly in or out. One technician, Krystian Kopka, described watching the March 2024 sunset as feeling like the Sun was setting in slow motion.

Ask a grown-up to help you look up when the Sun rises and sets.

Look at your town. Then look at Tromsø in June and December. What do you see?

NOAA's Solar Calculator, a free online tool from its Global Monitoring Laboratory, shows sunrise and sunset for any place on Earth and any date. Pick your own town and look up June 21 and December 21. Then try Tromsø, in Norway, on the same days.

What happens to sunrise and sunset in Tromsø in June? And in December? Then try a place right on the equator and compare the day lengths.

Use NOAA's Solar Calculator to build a data set. Choose four places along one line of longitude: one near the equator, one in the mid-latitudes, one just north of the Arctic Circle and one far north, such as Svalbard. For each, record sunrise and sunset on the first day of every month.

Plot day length against the month. Where does the curve hit 24 hours or 0 hours, and for how long? NOAA's glossary notes that its calculations correct only approximately for atmospheric refraction, so compare the results near the limits with the refraction values Store norske leksikon gives.