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tropic of cancer
The Tropic of Cancer, also referred to as the Northern Tropic, is the most northerly circle of latitude on the Earth at which the Sun may appear directly overhead at its culmination. This event occurs once per year, at the time of the Northern solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. As of , it lies at north of the Equator.〔(【引用サイトリンク】url=http://www.neoprogrammics.com/obliquity_of_the_ecliptic/ )〕
Its Southern Hemisphere counterpart, marking the most southerly position at which the Sun may appear directly overhead, is the Tropic of Capricorn. These tropics are two of the five major degree measures or major circles of latitude that mark maps of the Earth, besides the Arctic and Antarctic Circles and the Equator. The positions of these circles of latitude (other than the Equator) are dictated by the tilt of the Earth's axis of rotation relative to the plane of its orbit.
==Name==
The imaginary line is called the Tropic of Cancer because when the Sun reaches the zenith at this latitude, it is entering the astrological sign of Cancer (summer solstice in the northern hemisphere). When it was named, the Sun was also in the direction of the constellation Cancer (Latin for ''crab''). However, this is no longer true due to the precession of the equinoxes. According to International Astronomical Union boundaries, the Sun now is in Taurus at the June solstice. According to sidereal astrology, which divides the zodiac into 12 equal parts, the Sun is in Gemini at that time. However, according to tropical astrology, which divides the ecliptic in twelve 30° sectors, starting with the vernal equinox, the Sun is always entering Cancer at this time, as the Earth's axial tilt is most inclined towards the Sun. The word "tropic" itself comes from the Greek "trope (τροπή)", meaning turn (change of direction, or circumstances), inclination, referring to the fact that the sun appears to "turn back" at the solstices.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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