Latitude refers to the invisible horizontal lines that circle the earth. Four special parallels of latitude mark a specific area on the planet that has a unique relationship with the sun. In addition, these four lines mark the geographic boundaries of the frigid and torrid zones, which were first coined by Greek philosopher Aristotle. He stated these two zones were inhabitable because of the cold climate found in the frigid zone and the hot climate in the torrid zone. (See Reference 1 and Resource 5)
The Arctic Circle
The Arctic Circle runs at 66 degrees 33 minutes north latitude, making it the northernmost special parallel of latitude. This invisible circle is about 1,650 miles south of the North Pole, and marks the southernmost area on Earth where the sun doesn’t rise on the northern hemisphere's winter solstice. This event -- called polar night -- happens each year around December 21, and ranges from one day at the Arctic Circle, to six months at the North Pole. Further, the Arctic Circle marks the southernmost area where the sun doesn’t set on the northern hemisphere's summer solstice. This event also ranges from one day at the Arctic Circle, to six months at the North Pole, and generally occurs around June 21. The Arctic Circle runs through eight countries: Russia, Canada, the United States, Greenland, Norway, Finland, Sweden and Iceland.
The Tropic of Cancer
The Tropic of Cancer runs at 23 degrees 30 minutes north latitude and marks the northernmost area in which the sun passes directly overhead in its most vertical position at noon. This event occurs during the summer solstice for the northern hemisphere, which is usually June 21 or 22. The Tropic of Cancer also is the northern border for an area called the tropics. Because the sun consistently stays high in the sky year round, the tropics do not experience a range of seasonal climate changes. The Tropic of Cancer passes through 17 countries: Mexico, the Bahamas, Western Sahara, Mauritania, Mali, Algeria, Nigeria, Libya, Chad, Egypt, Saudi Arabia, United Arab Emirates, Oman, India, Bangladesh, Myanmar and China.
The Tropic of Capricorn
Running at 23 degrees 30 minutes south latitude of the Equator, the Tropic of Capricorn is the parallel that marks the southernmost area in which the sun passes directly overhead at noon. The sun appears in its most vertical position during the southern hemisphere's summer solstice, which is usually December 21 or 22. The parallel also makes up the southern border of the tropics area. The Tropic of Capricorn passes through 10 countries: Chile, Argentina, Paraguay, Brazil, Namibia, Botswana, South Africa, Mozambique, Madagascar and Australia.
The Antarctic Circle
The Antarctic Circle is the southernmost special parallel of latitude. It runs at 66 degrees 30 minutes south latitude and marks the northernmost point on Earth where the sun remains visible for 24 hours during the southern hemisphere’s summer solstice. This event, also called the midnight sun, generally occurs on December 21 or 22. In addition, the sun does not seem to rise during the southern hemisphere’s winter solstice, occurring on June 21 or 22. These events last about 24 hours at the northernmost point of the Antarctic Circle to six months at the South Pole. The Antarctic Circle passes through only one body of land: Antarctica.