G1Ch2Seasons

The video introduces two main aspects of seasons: the noticeable seasonal changes at a given location throughout the year and the underlying causes of these changes. It highlights two key observations: the sun's altitude and day length.

The sun's altitude refers to its height above the horizon at noon. In summer, the sun is higher, providing more direct insulation, while in winter, it's closer to the horizon, resulting in less direct insulation. Day length, the duration the sun is above the horizon, also varies seasonally, being longer in summer and shorter in winter, except at the equator where it's consistently 12 hours. These two factors combine to determine the amount of insulation received.

Contrary to a common misconception, seasons are not caused by the Earth's varying distance from the sun. The Earth is actually closest to the sun in January (winter in the Northern Hemisphere) and furthest in July (summer in the Northern Hemisphere). This difference in distance has a minor effect compared to other factors.

The primary cause of seasons is the Earth's axial tilt of 23.5 degrees relative to its orbital plane around the sun. This tilt remains constant as the Earth revolves around the sun. As a result, different parts of the Earth are tilted towards or away from the sun at different times of the year, leading to variations in direct sunlight.

The subsolar point is the location on Earth where the sun's rays are directly overhead at noon. This point migrates throughout the year. On the equinoxes (March and September), the subsolar point is on the equator, marking the beginning of spring and fall. On the solstices, it reaches its furthest points: the Tropic of Cancer (23.5 degrees North) in June (summer solstice) and the Tropic of Capricorn (23.5 degrees South) in December (winter solstice).

Visual aids demonstrate how the Earth's constant tilt, combined with its revolution, causes the subsolar point to shift. During the Northern Hemisphere's summer, the north pole is tilted towards the sun, placing the subsolar point at the Tropic of Cancer. In winter, it's tilted away, moving the subsolar point to the Tropic of Capricorn. This shift directly influences the intensity of sunlight received at different latitudes.

The spinning of the Earth on its tilted axis, while revolving around the sun, also explains the varying day lengths. At the poles, there can be six months of continuous light or darkness. At the equator, day and night are consistently 12 hours each. As one moves away from the equator, seasonality increasingly impacts day length, with extreme variations at higher latitudes where the sun may not set for days in summer or not rise for days in winter, though the sun's angle remains low.