什么是 Tropics and polar circles？

The two tropics are the lines around the globe, one north and one south of the equator, that mark how far from the equator the midday Sun can be straight overhead: the Tropic of Cancer in the north, the Tropic of Capricorn in the south. The two polar circles are the lines around the globe, near the poles, beyond which, in the middle of winter, the Sun does not rise for at least one day, and in the middle of summer it does not set: the Arctic Circle in the north, the Antarctic Circle in the south.