How the tropics of cancer and Capricorn were decided?

Answer 1

There is a summer solstice and a winter solstice. Summer solstice is the longest day of the year and winter solstice is the shortest day of the year in the Northern Hemisphere. Constellations are areas in the sky. At one time the sun was in the area occupied by the Constellation Cancer during Summer Solstice. At that point that particular latitude was named The Tropic of Cancer after that Constellation. At Winter Solstice the sun was in the area occupied by the Constellation Capricorn. That particular latitude was named, The Tropic of Capricorn after that Constellation.

Due to precession of the poles, the constellations of the Solstices has changed. The names of the tropics has not.

(Another interesting fact: Horoscopes list 12 constellations of the Zodiac. There are 13 and all the dates are wrong!)