Previous Year Question

The Earth rotates on its rotational axis. This axis is inclined by approximately 23.5° relative to the perpendicular to the plane of the Earth's orbit. Expressed another way, the Earth's axis makes an angle of approximately 66.5° with its orbital plane.

This axial tilt or obliquity is the fundamental reason for the occurrence of seasons on Earth. When the Northern Hemisphere is tilted towards the Sun, it receives more direct solar rays and experiences longer days, producing the summer season; at the same time, the Southern Hemisphere experiences winter. About six months later, the situation is reversed.

This angle is not completely constant. Over very long periods, the Earth's obliquity varies approximately between 22.1° and 24.5°, following a cycle of about 41,000 years. The present axial tilt is approximately 23.4°; therefore, textbooks and this older question commonly approximate it as 23.5°.