Scientific reasons for Earth’s seasons

Our planet’s tilt dictates cycle of summer and winter

Earth's tilt affects seasons. In this graphic, distances and sizes are not to scale.

The seasons are a powerful force in our lives. They affect the activities we do, the foods we crave, the clothes we wear — and quite often, the moods we are in. The seasons officially change once again Sunday, with summer beginning in the Northern Hemisphere and winter starting in the south.

What is it that causes the change in seasons?

The ability to predict the seasons — by tracking the rising and setting points of the sun throughout the year — was key to survival in ancient times. Babylonians, the Maya and other cultures developed complex systems for monitoring seasonal shifts. But it took centuries more to unravel the science behind the seasons.

Nicolai Copernicus (1473-1543) radically changed our understanding of astronomy when he proposed that the sun, not Earth, was the center of the solar system. This led to our modern understanding of the relationship between the sun and Earth.

We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of winter, spring, summer and fall.

Solstices and equinoxes CONTINUE