Big Science Ideas: Seasons are caused by the tilt of the Earth The angle of tilt does not change. As Earth revolves around the Sun, the position of the hemispheres in relation to the Sun changes. When the north pole is pointed toward the sun, the sun’s rays strike most directly (vertically) on the northern hemisphere. Direct rays of sunlight are more effective in producing light and heat than are indirect (slanting rays). There are two reasons why: 1) slanting rays must pass through a greater thickness of atmosphere than vertical rays and thus lose more of their heat; and 2) slanting rays are spread out over a greater area than vertical rays and are therefore less concentrated.