A Cycle of Seasons - What Causes Earth's Seasons?

The Earth as it looks from space during each season. From left to right, it's spring, summer, fall, and winter in the Northern Hemisphere. Image Credit: AHI image captured by the Japan Meteorological Agency’s Himawari-8 satellite (CC-BY 4.0)

In less than a week, on Sunday, June 21, summer will begin, at least in the Northern Hemisphere. But in the south, it will actually be the start of winter. Just how is it possible that two different places on the same planet could be experiencing different seasons at the same time?

It can be easy to assume that if it is hotter in the summer and colder in the winter that it must mean that the Sun is closer to the Earth in summer and farther in winter, but this is not the case. In fact, during the Northern Hemisphere’s summer, the Earth tends to be at its farthest point from the Sun on its orbital path. Instead, it’s the tilt of the Earth that causes these variations in temperature.

Imagine that the Sun and Earth are both on the same flat surface as the Earth orbits around the Sun. While the Earth is orbiting, it’s also rotating. This rotation is what gives us day and night. This makes the Earth a bit like a top that’s spinning on the surface as it moves around the Sun. However, if you spin a top, it probably won’t continue to spin with its handle pointing straight up all the time. Instead, it will soon have its handle point slightly towards the side so that the top is tilted at an angle other than 90° to the table beneath it. The Earth is the same way. The Earth’s axis of rotation is actually 23.4° off from being perfectly perpendicular to the imaginary plane it orbits on. This is likely due to a collision with another forming planet when the Earth was still new, which knocked the Earth slightly sideways. It is this tilt that is the actual culprit behind the seasons.

The tilt of the Earth's axis compared to the Sun means that sometimes one side of the Earth points more towards the Sun than the other, causing the seasons.

The Earth exhibits axial parallelism, which means that no matter how much it orbits around the Sun, its axis is always pointing in the same direction. As a result, for part of the year, the Northern Hemisphere will be leaning in towards the Sun, and for the other part, the same hemisphere will be leaning away. This is how the two hemispheres can experience the opposite seasons simultaneously, as when one is pointing towards the Sun, the other will be pointing away. It might be tempting to assume that the side of the Earth leaning towards the Sun is closer to it, and therefore will become hotter, but this is not the actual cause of the differences in temperatures between summer and winter.

It’s less about proximity to the Sun, and more about the angle that the light from the Sun arrives at Earth. When a portion of the Earth is tilted towards the Sun, the heat-bringing light is shining directly on it, but when it’s tilted away, the light hits at an angle which disperses the same amount of light over a larger area, decreasing the heat delivered. If Earth had little to no axial tilt, like Mercury, then it would experience no seasons, as sunlight would always hit the same parts of Earth in roughly the same amount of light, all year round.

Additionally, being tilted towards the Sun means that the Sun will appear higher in the sky. The higher the Sun appears in the sky, the earlier it will rise and the later it will set. This is why summer comes with longer days and winter with shorter. A solstice is defined as the day at which the Earth is pointed most towards the Sun. If it’s a summer solstice, the first day of summer, that means it will be accompanied by the longest time from sunrise to sunset, while a winter solstice, the first day of winter, will be the shortest. Just how long or short these daylight hours are depends on the location on Earth. At the North Pole, where this effect is most extreme, a summer solstice day lasts 24 hours, with the sun never setting, whereas a winter solstice will never see the sun rise.

But as one side of the Earth transitions from pointing towards the Sun to pointing away, there must be a transition point halfway through where it does neither. Those are the two equinoxes each year, which are the only two times a year where the Earth’s axis is not pointing towards or away from the Sun at all. As such, these days feature equal lengths of day and night, and are considered the first day of spring and fall. Each equinox is exactly halfway between the two solstices.

The seasons are one of the more obvious ways that life here on Earth is impacted by the mechanics of objects in outer space. Without our axial tilt, life on Earth would be very different.