As their relative locations change, so does the angle at which the sun's rays strike the earth.The span of the earth's surface that the sun directly strikes is very small; but it moves between the Tropics of Cancer and Capricorn, year in and year out. On the Summer Solstice, the sun shines directly on the Tropic of Cancer.
太陽直射的範圍小得可憐，不過是在北回歸線和南回歸線間往來移走，年復一年。