We all know that there are four seasons—spring, summer, autumn, and winter—in a year. However, in some regions, the seasons are categorized as cold, warm, dry, or wet. These variations are primarily caused by the angle at which sunlight strikes the Earth's surface at different latitudes. Places where sunlight strikes directly absorb more heat, making them relatively warmer, whereas areas where sunlight strikes at a more oblique angle absorb less heat, resulting in cooler temperatures.

What is the cause behind this phenomenon?

The Earth orbits the Sun in an elliptical path, which is nearly circular. The Sun occupies one of the focal points of this ellipse, which is not centered, causing the Earth's distance from the Sun to vary slightly over time—sometimes closer and sometimes farther away.

Many people believe that seasonal changes are caused by the Earth's elliptical orbit. The idea is that when the Earth is closer to the Sun, it gets warmer, and when farther away, it gets colder. However, in reality, the Earth's orbit is nearly circular and has minimal impact on weather patterns.

The primary cause of the changing seasons is actually the tilt of the Earth's axis, known in astronomy as its axial tilt or obliquity. The Earth tilts at an angle of approximately 23.5 degrees relative to its orbit around the Sun. As the Earth orbits the Sun—what we call revolution—the tilt of its axis causes sunlight to strike different parts of the Earth's surface at varying angles. Throughout the year, this angle of sunlight shifts between more direct (summer) and more oblique (winter) positions, creating the seasonal cycle of spring, summer, autumn, and winter.

How do we determine seasons?

According to the meteorological definition, each season spans three months and starts on the first day of the first month within that period. For instance, in the Northern Hemisphere, spring commences on March 1, summer on June 1, autumn on September 1, and winter on December 1.

According to the astronomical definition, seasons begin at either solstices (the two points in the year when the Sun is at its greatest distance from the equator) or equinoxes (when the Sun crosses the celestial equator). In the Northern Hemisphere, spring begins at the March equinox, summer at the June solstice, autumn at the September equinox, and winter at the December solstice.

Temperate, tropical, and polar

Most of the world's population resides in temperate regions, which lie between latitudes greater than 23.26° and less than 66.34°. Other latitudes encompass the tropics and polar zones. For instance, during the summer solstice, the sun is nearly overhead, resulting in the longest day of the year. Conversely, during the winter solstice, the sun reaches a maximum altitude on the horizon approximately 46.8° lower than during the summer solstice, marking the shortest day of the year.

The area spanning from the Tropic of Cancer to the Tropic of Capricorn is referred to as the tropics. Here, the sun's angle is nearly overhead throughout the year, providing consistent high levels of solar energy. As a result, temperatures in this region remain consistently warm year-round, without significant seasonal variations. Consequently, the tropics experience only one season—summer—without the distinct changes associated with spring, autumn, or winter.

The cold zones are situated in Earth's polar regions, where sunlight strikes at its shallowest angles, resulting in consistently cold climates, low temperatures year-round, and extensive ice and snow cover. The Antarctic Circle marks the southernmost latitude where the Sun can be seen directly overhead, specifically at 66°34′ south latitude. Similarly, the Arctic Circle is located at 66°34′ north latitude. Within these polar circles, unique phenomena occur: polar night and polar day. During summer in the Northern Hemisphere, the Arctic Circle experiences polar day, while the Antarctic Circle has polar night. Conversely, during summer in the Southern Hemisphere, the Arctic Circle undergoes polar night, while the Antarctic Circle enjoys polar day.