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tropics and polar circles

The middle line is the equator, the second and fourth line are the tropics, the first and fifth are the polar circles.

What is the relevance of these bands on Earth? Do these exist on other planets? What purpose do they serve?

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Good question, I wonder if these have analogues for Mars, which shares a similar axial tilt as Earth. –  user8 Sep 26 '13 at 9:17

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up vote 5 down vote accepted

The Tropics (Capricorn in the South and Cancer in the North) are the furthest point from the equator where the sun reaches a point directly overhead at any time during the year. The Arctic and Antarctic circles are the points at which the sun remains above the horizon for a full rotation of the Earth (i.e. 24 hours), commonly called "Midnight Sun". Any planet that has axial tilt while have similar lines.

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This page has a solar climate zone map for mars with the equator, the Tropic of Pisces, the Tropic of Virgo, and the Polar Circles depicted. –  called2voyage Sep 26 '13 at 22:00
    
@called2voyage good find. –  damned truths Sep 26 '13 at 23:04

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