US city just saw its last sunset until August

Whale bones form a monument to lost sailors in Utqiagvik, Alaska, overlooking the Arctic Ocean. (AP Photo/Laura Rauch)

For the next 81 days, the sun will go without setting in the northern Alaska city of Utqiagvik, formerly known as Barrow.

At 2 a.m. on May 11, the descended below the horizon in Utqiagvik just enough for the city to enter what's known as the civil twilight stage. Civil twilight occurs when the sun is about 6 degrees below the horizon and it's the first phase after sunset. During civil twilight, artificial lighting isn't necessary because sunlight is still illuminating the area, but the sun itself is no longer visible in the sky, according to the National Weather Service.

Sunset in Utqiagvik was short-lived, though.

Just 45 minutes later, at 2:45 a.m, the sun rose above the horizon again - and it won't descend below the horizon until the early-morning hours on Aug. 1.

There are four phases of daylight after a sunset, the first being civil twilight and after that comes nocturnal twilight, which occurs when the sun is 12 degrees below the horizon. During nocturnal twilight, outdoor light from the sun will diminish to the point where activities will need artificial lighting to continue.

The map radius tool in Utqiagvik (formerly Barrow), Alaska. (AP Photo/Earl Finkler/FILE)

The third stage is known as astronomical twilight and it occurs when the sun is 18 degrees below the horizon and there is near-total darkness. Astronomical twilight is followed by night, which is complete darkness.

People in Utqiagvik are presumably accustomed to seeing almost no darkness at this point of the calendar year. The city hasn't even experienced the nocturnal twilight phase since April 22 and has been without night since March 20, according to Time and Date AS.

The reason for such a unique occurrence stems from the location of Utqiagvik, which is situated north of the Arctic Circle and is the northernmost city in the United States.

As summer inches closer in the Northern Hemisphere, the direct rays of the sun approach closer to the Tropic of Cancer, which provides the Arctic Circle increasing amounts of sunshine during the summer.

Once the direct rays reach the Tropic of Cancer, summer officially begins.

The opposite phenomenon occurs in the winter for the Arctic Circle. Starting in November, Utqiagvik and other parts of the Arctic won't see the sun until it briefly makes it over the horizon in late January.

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