Because the earth turns daily on an axis that is tilted relative to the plane of the earth's yearly orbit around the sun, sunlight falls more intensely on different parts of the earth during the year.
(the earth; turns; T:daily; L:on an axis) (an axis; is tilted; relative to the plane of the earth's yearly orbit around the sun) (sunlight; falls more intensely; on different parts of the earth; T:during the year)
the earth an axis sunlight
turns is tilted falls more intensely
T:daily L:on an axis
relative to the plane of the earth's yearly orbit around the sun
on different parts of the earth T:during the year