What is the primary reason for the seasons on Earth?

The primary reason for the seasons on Earth is the tilt of Earth's rotational axis relative to its orbital plane. This axial tilt, which is approximately 23.5 degrees, causes different regions of the Earth to receive varying amounts of sunlight throughout the year as the planet orbits the Sun.

During summer in the Northern Hemisphere, the North Pole is tilted toward the Sun, resulting in longer days and more direct sunlight, which leads to warmer temperatures. Conversely, during winter, the North Pole is tilted away from the Sun, causing shorter days and less direct sunlight, which results in colder temperatures. The same effect occurs in the Southern Hemisphere but in reverse, with its summer occurring when the South Pole is tilted toward the Sun.

This axial tilt is the key factor that governs the changing seasons, rather than variations in distance from the Sun, the shape of the orbit, or the speed of rotation. While these other factors do have some effect on climate and temperature, they do not primarily determine the seasonal changes experienced on Earth.