The days are longer in summer than in winter due to the tilt of Earth's rotation axis compared to its orbital plane. In summer, the Northern Hemisphere is tilted towards the Sun, resulting in longer days, while in winter, it is the opposite.
Imagine a slightly tilted spinning top: that's exactly what our planet does all year round. The Earth indeed rotates around an axis tilted at about 23.5 degrees relative to the plane on which it orbits the Sun. This tilt is what causes longer or shorter days. When your hemisphere tilts towards the Sun, it's summer where you are and the days get longer because the Sun stays in the sky longer. Conversely, when your hemisphere moves away from the Sun during the year, the days become shorter: you're in the midst of winter. Without this tilt, there would be no seasons, and especially no long summer evenings in the sun!
The Earth orbits the Sun in a period of about 365 days, describing a slightly elongated ellipse rather than a perfect circle. This elliptical orbit means that the Earth sometimes passes a little closer to or a little farther from the Sun, but curiously, it is not this variable distance that explains the seasons. The true culprit is rather the Earth's tilted axis. As our planet moves along its orbital path, the Northern or Southern Hemisphere tilts more towards the Sun depending on its position in the orbit. In summer, therefore, the concerned hemisphere enjoys longer and more direct exposure to the Sun's rays; conversely, in winter, the same hemisphere is tilted away from the Sun, reducing the daily exposure time to its rays.
The solstices and equinoxes simply mark the seasons based on the Earth's position relative to the Sun. During the summer solstice around June 21, the Northern Hemisphere tilts towards the Sun, making it the time when the days are the longest of the year. Conversely, at the winter solstice around December 21, the Sun primarily illuminates the Southern Hemisphere, and the days become noticeably shorter in the Northern Hemisphere. During the equinoxes (around March 21 and September 23), the Earth is oriented in a balanced way relative to the Sun. As a result, everywhere on the globe, day and night last approximately the same duration. These key phases clearly mark how the length of day varies throughout the year according to our planet's position.
Depending on the latitude where you are, the difference between the length of days in summer and winter varies greatly. Near the equator, the difference is minimal all year round: you will hardly notice any change, with days and nights always hovering around twelve hours. But the closer you get to the poles, the more extreme it becomes. There, you will experience periods when the sun never fully sets (midnight sun), and conversely, periods when it doesn’t rise at all (polar night). In short, the higher your latitude (the farther you are from the equator), the more spectacular the alternation between day and night becomes throughout the seasons.
The way sunlight hits the Earth changes depending on where you are and the seasons. When the rays arrive at a straighter angle (close to vertical), they are more concentrated and heat the surface more effectively. Conversely, when the sun shines at a slanting angle, its rays spread over a larger area and heat less intensely. This explains why in summer, with a more direct angle, it is warm and the days seem brighter. In winter, the more inclined angle scatters the rays more: the days are cooler and less bright. It's all a matter of simple geometry between the position of the Earth and the Sun.
The current tilt of the Earth's axis is approximately 23.5°, but it changes slowly over time. This cyclical variation, called obliquity, affects the intensity and duration of the seasons on Earth over the millennia.
At the equator, the length of the day hardly varies throughout the year, with approximately 12 hours of daylight and 12 hours of night. The further one moves away from the equator towards the poles, the greater the variation in day length becomes according to the seasons.
During the solstices at the polar circles (Arctic and Antarctic), there are phenomena of continuous days or nights lasting 24 hours. This is respectively called the 'midnight sun' or the 'polar night'.
The Earth reaches its farthest point from the Sun (aphelion) at the beginning of July, while it is summer in the Northern Hemisphere. This paradox clearly shows that seasonal variations are not due to the Earth-Sun distance but rather to the tilt of the Earth's axis.
Although the Earth is slightly farther from the Sun in summer in the Northern Hemisphere, temperatures rise because sunlight strikes at a more direct angle, concentrating solar energy over a smaller area and leading to higher temperatures.
At the poles, due to the tilt of the Earth's axis, the sun remains constantly below (or above) the horizon for nearly 6 consecutive months. This explains the long polar night in winter and the polar day in summer.
The summer solstice marks the longest day of the year in the Northern Hemisphere (around June 21). After this date, the days gradually begin to shorten as the tilt of the Earth's axis causes the Sun to reach a slightly lower height in the sky each day, thereby reducing the daily duration of sunlight until the winter solstice.
In theory, the equinoxes should mean 12 hours of sunlight and 12 hours of night everywhere on Earth. However, in practice, the Earth's atmosphere and light refraction slightly extend the duration of the day, making the days slightly longer than 12 hours during the equinoxes.
Contrary to popular belief, the seasons are not caused by the varying distance between the Earth and the Sun, but rather by the tilt of the Earth's axis. The hemisphere tilted towards the sun experiences summer with longer days, while the hemisphere tilted away experiences winter, with shorter days.
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