As December comes to a close, Earth will move forward in its annual switch of the seasons.

Sunday, December 21st, just after 10 a.m., marks the year’s winter solstice, or one of two points where Earth is most tilted on its axis. In this case, the Earth will basically “lean back”, allowing the Southern Hemisphere to be more tilted towards the Sun. It is also the kickoff to the winter season.

Not all folks see winter weather during this time, however. This marks the start of summer in the Southern Hemisphere, where people in Australia, southern South America, and Africa experience their warmest weather of the year. For some, though, the winter solstice arrives in the midst of a dark, cold time.

Polar Night peaks on this day in the Northern Hemisphere. The phenomenon gives those living above the Polar Circles (66.33 degrees) the exclusive feat of getting zero hours of daylight. 

Polar nights (and their antithesis, the Midnight sun) stem from the way the Sun can effectively cover Earth. As the Earth is a sphere, it can only shine on half of the globe at one time. During the equinoxes (fall and spring), the Earth has zero tilt, giving everyone in each hemisphere an equal 12 hours of sunlight. However, as the Northern Hemisphere begins to tilt away from the Sun after the autumnal equinox, the Sun shines on the Southern Hemisphere in a greater proportion, shielding those far north in the Northern Hemisphere. 

As maximum tilt occurs on the Winter Solstice, still favoring the Southern Hemisphere, those above a certain latitude would not get to see the Sun rise at all. Those right on the circle may only get a day or two of Polar Night, while those on either pole are guaranteed six straight months of Polar Night.

This is why your latitude on Earth is the biggest teller for how long your days will be. Those closer to the equator, who will always be favorably tilted towards the Sun, will tend to stick to 12-hour days, give or take some minutes. People around the poles will see dramatic swings of daylight around the spring or fall equinoxes. Back here at home, Youngstown’s place at 41° North puts the Valley in a sort of “Goldilocks” position, where there is a decent variation in day length between 9 and 15.5 hours, without treading too far into the extremes.

The positions of the Arctic and Antarctic Circles (the Hemispheric names of these Polar Circles) can vary slightly based on long-term fluctuations. Currently, the Polar Circles are at 66°33′50.7″ from the Equator. Its latitude can fluctuate by up to 2° over 41,000 years. For now, the circles are drifting poleward at a speed of about 48 ft per year.

The term “Polar Night” may be a little confusing for some, however, as most locations aren’t fully dark throughout the entire 24-hour day. In places closer to the polar circles and equinoxes, some of their day might actually be spent in twilight, rather than full night. Twilight takes three official forms, depending on how far the Sun is below the horizon. Civil Twilight occurs when the sun is less than 6° below the horizon, and is usually the first 30 or so minutes of light after the Sun gets below the horizon. Nautical twilight is next, when the Sun gets between 6° and 12° below the horizon. At this point, light is still faint, but things can get very hard to see with the naked eye. It’s at this point that bright stars become visible, as well. Astronomical sunset occurs when the sun is between 12° and 18° below the horizon, the last bits of light before full darkness.

To experience true polar darkness- no twilight for a full 24 hours- you’d need to get to at least 84°34′ from the equator. The only settlement poleward from that latitude? The Amundsen-Scott research station at the South Pole, which gets this stellar recognition for almost three months.