Why are there only four seasons on earth? Why are there four seasons? Why do different seasons occur at the same time in the Northern and Southern Hemispheres?
Some assume that the changing distance of our planet from the sun causes change in the seasons. That is logical, but it is not the case, for the Earth. Almost everyone enjoys the change of seasons on Earth: from winter to spring, from summer to autumn. But, why do our stations change?
Some assume that the changing distance of our planet from the sun causes change in the seasons. That is logical, but it is not the case, for the Earth. Instead, Earth has stations because our planet’s axis of rotation is tilted at an angle of 23.5 degrees to our orbital plane, the plane of the Earth’s orbit around the sun.
The inclination on the Earth’s axis is called obliquity on the part of scientists.
In the course of a year, the inclination angle does not change. In other words, the north axis of the Earth always points in the same direction in space. At this moment, that direction is more or less towards the star that we call Polaris, the polar star. But the orientation of the Earth’s inclination to the sun, our source of light and heat, changes as we orbit around the sun. In other words, the Northern Hemisphere is oriented towards the sun during the middle of the year and away from the sun towards the other half. The same is true for the Southern Hemisphere.
When the northern hemisphere is oriented towards the sun, that region of the Earth heats up due to the corresponding increase in solar radiation. The sun’s rays hit that part of the Earth at a more direct angle. It’s summer.
When the northern hemisphere is oriented away from the sun, the sun’s rays are less direct and that part of the Earth cools. It is winter.
The seasons in the southern hemisphere occur at times of the year opposite to those of the northern hemisphere. North summer = southern winter.
The tilt in the Earth’s axis is strongly influenced by the way in which the mass is distributed over the planet. Large amounts of landmass and layers of ice in the northern hemisphere make the Earth very heavy. An analogy of obliquity is to imagine what would happen if you had to spin a ball with a gum stuck near the top. The extra weight would cause the ball to tilt when turned.
During long periods of geological time, the angle of obliquity of the Earth oscillates between 21.4 and 24.4 degrees. This cycle lasts approximately 41,000 years and is believed to play a key role in the formation of the ice ages, a scientific theory proposed by Milutin Milankovitch in 1930.
Earth is currently decreasing in obliquity. The decrease in obliquity can prepare the ground for more moderate seasons (colder summers and warmer winters) while increases in obliquity create more extreme seasons (hotter summers and colder winters). Glaciers tend to grow when the Earth has many cool summers that do not melt the winter snows. Remember, we are talking about a cycle of 41,000 years here, so these changes in obliquity are not the main driver of Earth’s climate in the next century. Temperatures on Earth are influenced not only by obliquity, but also by many more factors that drive our complex climate system and the global temperatures we experience year after year.
Other planets in our solar system also lean to varying degrees. Uranus turns almost sideways to 97 degrees and has extreme seasons. The axial inclination on Venus is 177.3 degrees. Therefore, Venus has very little in the way of the seasons.
The distance from the Earth to the sun changes throughout the year, and it is logical to assume that an increase or decrease in the distance of a solar planet could cause a cyclic change in the seasons. But, in the case of our planet, this change is too small to cause this change.
Our stations change due to the angle of inclination of our planet – 23.5 degrees – in relation to our orbit around the sun. If the Earth did not lean at all, but rotate exactly upright relative to our orbit around the Sun, there would be minor temperature variations during each year as the Earth moved a little closer to the Sun and then a little further away. And there would be differences in temperature from the equatorial region of the Earth to the poles. But, without the Earth’s inclination, we would lack the wonderful seasonal changes of the Earth and our association of them with the different seasons, associating a fresh sensation in the air with the spring, for example.
It is easy to imagine a planet that has a more pronounced change in its distance from its star as the planet orbits the star. Some extrasolar planets, planets orbiting distant stars, have found themselves with more extreme orbits. And even in our own solar system, for example, the planet Mars has a more elliptical orbit than Earth. Its distance from the sun changes more dramatically throughout its year than that of the Earth, and the change in distance from Mars to the sun does cause more pronounced cyclical changes in this desert red world.
In short: it is logical to assume that the changing distance of our planet from the sun causes change in the seasons. But the distance from Earth to the sun does not change enough to cause seasonal differences. Instead, our seasons change because the Earth is tilted on its axis, and the angle of inclination makes the northern and southern hemisphere exchange places throughout the year to receive light and heat from the sun more directly.
