Tropic of Cancer, the parallel of latitude, or imaginary line around the earth, that marks the northern limit of the tropics or Torrid Zone, a region where the climate is consistently warm. The Tropic of Cancer lies 23°27’ north of the equator and is the northern solstice—that is, it is the northernmost point where the sun appears directly overhead at least one day a year.
The sun reaches this apparent position at noon on or about June 22 of each year; at this time it appears at its highest point in the sky as seen from the northern hemisphere, and at its lowest point as seen from the southern hemisphere.
This date marks the start of summer for the northern hemisphere and is therefore called the summer solstice. In the southern hemisphere, however, the seasons are reversed; this date marks the start of winter in the southern hemisphere, and it is known there as the winter solstice. For the next six months the sun at noon appears lower each day in the northern hemisphere and higher each day in the southern hemisphere.
Cancer is the Latin word for crab, and is the name of one of the 12 sectors that ancient astrologers marked off in the zodiac, the band of sky from 8° north to 8° south of the equator. In the 2nd century bc Greek astronomer Hipparchus observed that the sun appeared to be within the boundaries of Cancer at the time of the summer solstice in the northern hemisphere. The Tropic of Cancer divides the tropics from the North Temperate Zone and crosses Mexico, the Caribbean Sea, the Sahara Desert of Africa, central India, southern China, and the Pacific Ocean just north of Hawaii.