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- the parallel of latitude 23°26' north (tropic of Cancer) or south (tropic of Capricorn) of the equator回归线（纬度均为23°26'的赤道北、南两条平行线： 北回归线 [tropic of Caucer]和南回归线 [tropic of Capricorn]）。
1.1
- Astronomy each of two corresponding circles on the celestial sphere where the sun appears to turn after reaching its greatest declination, marking the northern and southern limits of the ecliptic【天文】天球的回归线。
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the tropics
the region between the tropics of Cancer and Capricorn热带（北、南回归线之间的地带）。
词源
late Middle English (denoting the point on the ecliptic reached by the sun at the solstice)： via Latin from Greek tropikos, from tropē 'turning', from trepein 'to turn'.