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The Tropic of Cancer , which is also referred to as the Northern Tropic , is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice , when the Northern Hemisphere is tilted toward the Sun to its maximum extent. Its Southern Hemisphere counterpart, marking the most southerly position at which the Sun can be directly overhead, is the Tropic of Capricorn. These tropics are two of the five major circles of latitude that mark maps of Earth, the others being the Arctic and Antarctic Circles and the Equator. The positions of these two circles of latitude relative to the Equator are dictated by the tilt of Earth's axis of rotation relative to the plane of its orbit , and since the tilt changes, the location of these two circles also changes. When this line of latitude was named in the last centuries BC, the Sun was in the constellation Cancer Latin for crab at the June solstice , the time each year that the Sun reaches its zenith at this latitude. Due to the precession of the equinoxes , this is no longer the case; today the Sun is in Taurus at the June solstice. The Tropic of Cancer's position is not fixed, but constantly changes because of a slight wobble in the Earth's longitudinal alignment relative to the ecliptic , the plane in which the Earth orbits around the Sun. Earth's axial tilt varies over a 41,year period from This wobble means that the Tropic of Cancer is currently drifting southward at a rate of almost half an arcsecond 0.
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 Earth's most northerly circle of latitude where the sun is located directly above at noon during the summer solstice. 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 imaginary lines known as parallels that circle Earth from east to west that run surprise! 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?
At the summer solstice in the Northern Hemisphere, around June 21, the Sun attains its greatest declination north and is directly over the Tropic of Cancer. At that time the Sun appears in the constellation Gemini , but, much earlier in history, it lay in the constellation Cancer , thereby resulting in the designation Tropic of Cancer. See Tropic of Capricorn. Tropic of Cancer. Article Media. Info Print Cite. Submit Feedback. Thank you for your feedback.