The seasons are predictable changes in weather, temperature, and the amount of daylight we experience throughout the year. These shifts influence everything from clothing choices to when farmers plant their crops. Our planet’s annual movement through space, combined with its unique physical orientation, is responsible for this recurring cycle of spring, summer, autumn, and winter. This movement explains why one part of the world might be experiencing warmth while another faces cold weather.
The Earth’s Constant Tilt
The Earth spins on an imaginary line, called the axis of rotation, that runs through its center from the North Pole to the South Pole. This axis is constantly tilted at an angle of approximately 23.5 degrees. This axial tilt is the single most important factor in causing the seasons.
As the Earth travels around the Sun, its axis remains pointed in the same direction in space, always toward the North Star (Polaris). This fixed orientation is similar to how a spinning top maintains its lean. This stable direction of the tilt means that over the course of the year, different parts of the planet are exposed to the Sun in different ways.
Traveling Around the Sun
Our planet travels along a fixed path around the Sun, a journey called an orbit or revolution. Completing one full orbit takes about 365.25 days, which defines the length of one year. The path of this orbit is not a perfect circle, but a shape called an ellipse.
Due to this slightly oval shape, the Earth is closer to the Sun at one point in its orbit and farther away at another. However, this difference in distance is minimal, varying by only about three percent throughout the year. This small change in distance does not determine the seasons; the Northern Hemisphere, for example, experiences winter when the Earth is closest to the Sun.
How Tilt Changes Our Sunlight
The combination of the Earth’s constant tilt and its orbit creates two main effects that cause temperature changes. The primary effect involves the angle at which sunlight strikes the surface. When a hemisphere is tilted toward the Sun, the sunlight hits that area more directly, nearly straight-on. This concentrated light delivers more energy to a smaller area, causing the ground and atmosphere to warm up significantly.
When a hemisphere is tilted away from the Sun, the sunlight strikes the surface at a more slanted angle. This slanted light is spread out over a much larger area, making the energy less intense and less effective at heating the ground. Slanted rays also pass through a greater thickness of the atmosphere, which scatters some of the energy before it reaches the surface.
The second effect is the change in the duration of daylight hours. The hemisphere tilted toward the Sun spends more time in daylight, resulting in longer days and shorter nights. More daylight hours allow the Earth to absorb more heat from the Sun, contributing to the higher temperatures of summer. When a hemisphere is tilted away, the days are shorter, and the longer nights allow accumulated heat to escape, leading to the colder temperatures of winter.
Defining the Start of Each Season
We use specific astronomical markers—the solstices and the equinoxes—to pinpoint the official start of each season. The solstices mark the moments when one of the Earth’s hemispheres is tilted maximally toward or away from the Sun. The summer solstice has the longest period of daylight, while the winter solstice has the shortest.
The equinoxes occur halfway between the solstices, marking the beginning of spring and autumn. During an equinox, the Earth’s tilt is neutral, meaning neither hemisphere is leaning toward or away from the Sun. This alignment results in the duration of daytime and nighttime being nearly equal all over the world.