It occurs due to the the orientation of Earth’s tilt with respect to the sun which is the source of light and warmth .The same light and warmth change as Earth orbit the sun. In other words, the Northern Hemisphere is oriented toward the sun for half of the year and away from the sun for the other half. The same is true of the Southern Hemisphere.We can say that When the Northern Hemisphere is oriented toward the sun, that region of Earth warms because of the corresponding increase in solar radiation. The sun’s rays are striking that part of Earth at a more direct angle resulting into the summer.When the Northern Hemisphere is oriented away from the sun, the sun’s rays are less direct, and that part of Earth cools It’s winter ,the same phenomenon of Seasons in the southern hemisphere occur at opposite times of the year from those in the northern hemisphere. Northern summer = southern winter and vice versa.