Antarctic circle
The 'círculo polar antártico' is Spanish for 'Antarctic Circle'. The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. It is the parallel of latitude that runs 66°33′46.1″ south of the Equator. Within the Antarctic Circle, the sun does not set for at least one day each year (the December solstice) and does not rise for at least one day each year (the June solstice). The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone.
When you travel to Antarctica, you will enter the Antarctic polar circle.
This sentence uses the future tense to describe an event that will happen in the future. It describes a hypothetical journey to Antarctica, which would naturally involve crossing the Antarctic polar circle.
The Antarctic polar circle demarcates the part of the world that has at least one day of total darkness each year.
This sentence discusses a geographical concept related to the Antarctic polar circle. It refers to the phenomenon of total darkness for 24 hours, which occurs within the boundaries of the Antarctic polar circle once a year.
In the middle of the Antarctic polar circle, temperatures can be extremely low.
This sentence uses the present tense to describe a general fact about the conditions in the Antarctic polar circle. Because of its location at the extreme south of the earth, temperatures within the Antarctic polar circle can drop significantly.