Dear Student,
a. The earth's axis is tilted about 23.5 degrees, so the Earth is never straight up and down compared to the Sun.
b. When the earth's axis points towards the sun, it is summer for that hemisphere. When the earth's axis points away, it is winters
c. Therefore, In the month of June, the sun is directly overhead at the north of equator as Northern hemisphere is tilted towards the sun and in December, the noon sun is overhead in the south of equator, as the southern hemisphere is tilted towards the sun.
d. Thus, both northern and southern hemisphere experience each season for half a year.
Regards