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Why Is the Tropic of Capricorn Important?

Tropic of Capricorn
The Tropic of Capricorn sign in Namibia, Africa. Between Sossusvlei and Walvis Bay in Namibia, two colorful signs, one on each side of the road, mark the spot where the Tropic of Capricorn crosses. paranyu pithayarungsarit/Getty Images

In 1939, author Henry Miller wrote a semiautobiographical novel that created a whole lot of controversy. The book in question? "Tropic of Capricorn," which was banned in the United States until 1961 because of its explicit sexual content. The novel chronicles the protagonist's life, including a job at the "Cosmodemonic Telegraph Company of North America," which you can suspect influenced the book's title. But "Tropic of Capricorn" (as well as Miller's other famed book, "Tropic of Cancer") have roots outside the literary world and in the stars.

Otherwise known as the Southern tropic, the Tropic of Capricorn is one of five major circles of latitude that you'll find on a map or globe. These latitudes are the imaginary east-west circles that indicate positions on Earth, when coupled with lines of longitude, which are imaginary circles intersecting the North and South poles and equator. The Tropics of Capricorn and Cancer were delineated because they are both places within the hemispheres where it's possible for the sun to be directly overhead. For ancient travelers who used the heavens to guide their way, these were crucial demarcation lines. The five major lines of latitude mark specific points on Earth:

  • The equator is probably the most well-known circle of latitude, and it's situated at zero degrees latitude. It has a circumference of about 25,000 miles (40,233 kilometers), and it divides the planet into the Northern and Southern hemispheres. The latitudes of other circles are determined by their distance north or south of the equator.
  • The Arctic Circle is located at approximately 66.5 degrees north latitude, or 66.5 degrees north of the equator.
  • The Antarctic Circle is located at approximately 66.5 degrees south latitude, or 66.5 degrees south of the equator.
  • The Tropic of Cancer is located at approximately 23.5 degrees north latitude, or 23.5 degrees north of the equator.
Tropics
This world map shows the Tropics of Capricorn and Cancer and the equator. Not shown are the other two major latitude lines: The Arctic Circle runs around the north latitude, while the Antarctic Circle runs around the south latitude.
Jennifer Thermes/Getty Images

That brings us to No. 5: The Tropic of Capricorn. Located at approximately 23.5 degrees south latitude, or 23.5 degrees south of the equator, this circle marks the southernmost point on Earth where the sun is directly overhead at noon. To be even more specific, it marks where the sun is overhead at noon on Dec. 21, which marks the beginning of summer in the Southern Hemisphere.

So why the astrological reference? The lines of latitude were named a long time ago — about 2,000 years ago, to be exact. When the Tropic of Capricorn was named, the sun happened to be in the constellation Capricorn during the December solstice (centuries later, the sun no longer sits in that constellation at that time of year). And as far as the "tropics" reference, the geographical sites are significant because they are the places on Earth where it's actually possible for the sun to sit directly overhead.

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