Why Is the Tropic of Cancer Important?

By: Michelle Konstantinovsky  | 
Tropic of Cancer
The Tropic of Cancer sign near Bhopal, India. The Tropic of Cancer is marked as a yellow line at 23.5 degrees north latitude on the Bhopal-Sanchi Road. Yann Forget/Wikimedia Commons (CC BY-SA 3.0)

"It was because to me, cancer symbolizes the disease of civilization, the endpoint of the wrong path, the necessity to change course radically, to start completely over from scratch." That's how author Henry Miller explained the title of his controversial 1934 novel, "Tropic of Cancer." The book, set in late 1920s and early 1930s France, focuses on Miller's life as a struggling writer. While his inspiration for the title is perfectly fitting, some readers are surprised to find the subject material completely unrelated to the other Tropic of Cancer. So, what is the Tropic of Cancer and why is it important?

Let's start with the basics: Latitude is a measurement of distance north or south of the equator, the imaginary line that encircles the middle of the planet. The equator's latitude is 0 degrees, and there are 180 imaginary lines (known as parallels) that circle Earth from east to west that run (surprise!) parallel to the equator. A "circle of latitude" refers to the imaginary ring that links all the points on a shared parallel. The latitude of the North Pole? Ninety degrees north. The South Pole? Ninety degrees south.


Circles of Latitude: Tropics of Cancer and Capricorn

While you've probably heard of the North and South poles, you may not be as familiar with the other circles of latitude. And while there aren't proper names for all of them, there are five major ones to know: the equator, the Tropics of Cancer and Capricorn, and the Arctic and Antarctic circles. The Tropics of Capricorn and Cancer were delineated because they are both places within the hemisphere 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. That's part of the reason the Tropic of Cancer is important.

This world map points out the Tropics of Capricorn and Cancer. Not shown are the other two major latitude lines: the Arctic Circle, which runs around the north latitude, and the Antarctic Circle, which runs around the south latitude.
Jennifer Thermes/Getty Images


Part of the Earth's Tropical Zone

Located at approximately 23.5 degrees north latitude (i.e., 23.5 degrees north of the equator), the Tropic of Cancer is the line of latitude that's the northern boundary of the area referred to as the tropics. The tropics account for about 36 percent of Earth, and about a third of the world's population lives in this area that includes the equator and parts of North America, South America, Africa, Asia and Australia.


Summer Solstice: When the Sun Appears Directly Overhead

During the summer solstice (around June 21), the sun is located immediately overhead the Tropic of Cancer, making it the farthest point north on Earth where the sun is located directly above at noon. When the Tropic of Cancer was first named about 2,000 years ago, the sun was pointed in the direction of the constellation of Cancer during the June solstice. The name stuck, even though the location of the sun has shifted and now resides in the constellation of Taurus during that time of year.


Tropic of Cancer FAQs

What is the Tropic of Cancer?
The Tropic of Cancer is the most northerly of Earth’s latitudes, where the sun appears straight overhead at noon (on the summer or June solstice). The Tropic of Cancer forms the northern boundary of the area referred to as the tropics.
Where is the Tropic of Cancer?
The Tropic of Cancer is located at approximately 23.5 degrees north latitude (i.e., 23.5 degrees north of the equator).
How many countries does the Tropic of Cancer pass through?
The Tropic of Cancer goes through 16 countries, six water bodies and three continents. It cannot be seen because it’s an imaginary line. Some countries through which the Tropic of Cancer passes are Libya, Niger, Saudi Arabia, UAE, India, the Bahamas and China.
Why was the book "Tropic of Cancer" banned?
"Tropic of Cancer" is a 1934 novel by Henry Miller that was banned in the United States because of its sexual content. It was eventually published there in 1961.