We have seasons on Earth because Earth's axis is tilted about 23.4 degrees relative to its orbit around the Sun. This tilt causes different parts of Earth to receive varying amounts of sunlight throughout the year. As Earth orbits the Sun, its tilted axis always points in the same direction, so sometimes the Northern Hemisphere is tilted toward the Sun (summer there) and sometimes the Southern Hemisphere is tilted toward the Sun (summer there). This variation in sunlight intensity and day length causes the seasons, not the distance from the Sun, which only changes slightly during Earth's elliptical orbit.
How the Tilt Causes Seasons
- When a hemisphere tilts toward the Sun, it receives more direct sunlight and experiences warmer temperatures (summer).
- When a hemisphere tilts away from the Sun, the sunlight is less direct and temperatures are cooler (winter).
- The equinoxes occur when the tilt is such that day and night are approximately equal in length worldwide.
- The opposite hemisphere experiences the opposite season at the same time.
Why Distance from the Sun Is Not the Cause
- Earth’s orbit is elliptical but the change in distance to the Sun is minor and does not cause the seasons.
- For example, the Northern Hemisphere has winter when Earth is closest to the Sun, and summer when it is farthest.
Thus, the tilt of Earth's axis relative to its orbital plane around the Sun is the fundamental reason for the seasons experienced on Earth.
