"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. And 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, otherwise known as the Earth's most northerly circle of latitude.
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. That latitude of the North Pole? Ninety degrees north. The South Pole? Ninety degrees south.
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.
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.
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.