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The Tropic of Cancer is the circle of latitude on the Earth that is the most northerly position at which the Sun may appear directly overhead at its zenith. It only happens once a year, at the June solstice. In the Southern Hemisphere the most southerly position is the Tropic of Capricorn. These two positions are part of the five major degree measures along with the Arctic Circle, the Antarctic Circle, and the equator. Each of these positions of latitude(not the Equator) are dictated by the tilt of the Earth’s axis of rotation relative to the plane of orbit.
It is called the Tropic of Cancer because, at the time that it was named, the Sun was in the direction of the Cancer constellation. The precession of the equinox has changed that and the Sun is now in the direction of Taurus at the June solstice or in Gemini according to astrology. It currently lies 23°26?16? north of the equator. It is not in a fixed position. It is presently drifting south at the rate of almost half a second of latitude per year.
According to the Federation Aeronautique Internationale rules, in order for a flight to qualify as being round-the-world, it must cover a distance no less than the length of the Tropic of Cancer, cross all of the meridians, and end on the same airfield where it started. This length is set to be 36,787.559 kilometers, but varies because of the variations of the Tropic of Cancer.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.