Some believe that the shift in seasons is caused by our planet's fluctuating distance from the sun. That makes sense, but it's not the case for Earth. Seasons exist on Earth because our planet's axis of rotation is tilted at an angle of 23.5 degrees relative to our orbital plane, or the plane of Earth's orbit around the sun. As a result, portions of Earth are always facing the sun, no matter where we find ourselves in our orbit.
Seasons affect all life on Earth because they cause changes in the amount of sunlight and heat received by different parts of the planet. For example, during spring tide (the high point in the moon's monthly cycle), areas as far south as Florida and Texas experience their highest levels of sunlight ever recorded during a given year. By contrast, near autumn equinox, regions as far north as Canada and Alaska experience their lowest levels of sunlight ever recorded during a given year. The difference between maximum and minimum levels of sunlight received by Earth's surface is called its "insolation."
You might think that every part of Earth experiences both extremes of insolation, but this is not true. Because of the tilt of Earth's axis, half of the planet is constantly shielded from direct sunlight. During winter, the portion of Earth that is shielded sends signals to the other half, telling it to grow plants for food. This is why we see evidence of vegetation change every year even though the amount of sunlight reaching the ground remains roughly the same throughout most of the world.
Scientists refer to the tilt in the Earth's axis as its obliquity. The angle of tilt does not change throughout the course of a year. However, due to gravitational interactions with other objects in the solar system, including Jupiter and Saturn, Earth's orbit changes slightly in size each year.
As Earth orbits the sun, it travels along an elliptical path. During part of this journey, we are in direct sunlight, while during other parts of the orbit we are in darkness. The time spent in darkness varies from about 3 hours at perihelion, the closest point to the sun, to about 5 hours at aphelion, the farthest point from the sun. These periods of daylight and darkness affect how much heat is absorbed by the surface below.
At any given place on Earth, average daily temperatures vary depending on when you reach them. At the beginning of spring, when the days are getting longer, the temperature rises. By late summer, when the days are getting shorter, the temperature drops. This is why locations near the equator experience only one season - the warmest season is called "the dry season". The coldest season is called "the wet season".
The seasons fluctuate on Earth as a result of the tilted axis and the earth's movement around the sun. If the seasons had not changed, the axis would not have been slanted. As a result, extreme cold will prevail in the poles, while excessive heat will prevail towards the equator. The only comfort for those living at the poles would be the fact that winter is less severe than summer.
At the moment of spring, when daylight hours increase and the temperature rises, plants expand their leaves and grow pollen grains. This is how plants reproduce themselves. During flowering periods, insects such as bees make use of this by fertilizing flowers with their loads of pollen. After pollination, the flower wilts and falls off, thus preventing other seeds from growing inside the fruit. When plants shed their old leaves and grow new ones, they use up much of their stored energy. This is why plants go through these changes annually: to regain their strength for another season.
People used to think that winter was forever, but now we know that it does not last forever. At some point in time, spring will come again. So will summer, fall, and winter. This is how cycles always return to balance.
There are many examples of this phenomenon on Earth. For example, every seven years or so, all the trees on Mount Fuji erupt into bloom almost simultaneously.