The Earth's climate is an amazing topic to research and particularly the polar influences which condition a largely Equatorial climate. As the Earth forward motion through space turns the entire surface of the planet, the circumference where the Sun is in view or absent either expands and contracts in response to that rotation -

http://www.msss.com/mars_images/moc/...mericas250.jpg

The Arctic and Antarctic circles are merely the maximum circumference for this seesawing geometry but neglected because academics refuse to budge from their tilting Earth notion to explain the seasons or have some awful pivoting circle of illumination off the Equator thereby sadly misusing imaging -

https://apod.nasa.gov/apod/ap140319.html

Today is the day they say it is polar night at the South Pole however the moon is so bright presently that it hardly appears that way even though the Sun has been absent from view since March 21st.

All good stuff and none of it touched by human intellect, after all, who has accepted that the Earth has two distinct rotations to the Sun ?.