When a planet has perfect alignment between the axis it orbits on and the rotational axis, the amount of sunlight it receives is fixed as it orbits around the Sun.
So, the Northern Hemisphere experiences more intense sunlight during the summer, when the Sun is positioned more directly above the Northern Hemisphere. Then, as the Earth continues to orbit around the Sun, the amount of sunlight the Northern Hemisphere receives gradually decreases as the Northern Hemisphere tilts away from the Sun. This causes winter.
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