The Earth's seasons are the result of the Earth's rotation around the Sun and the tilt of its axis relative to the ecliptic. The seasons mark the time of year when the Earth experiences weather and ecological changes. The seasons in different regions in the Earth are characterized by variations in the intensity of solar radiation, hitting the surface of the Earth. The Earth's temperate and sub-polar regions experience all four known seasons; winter, spring, summer and autumn, while hotter regions, such as the tropics, may only experience two or three seasons, a rainy, a dry and a mild season.
The Earth revolves around the Sun in the lapse time of 365 days, during which it axis subtends an angle of 23.5 degrees with respect to the plane of the ecliptic. This tilt causes regions of the Earth to receive the Sun's rays at different intensities throughout the year, thus, during the summer, when the tilt of the Earth is facing the Sun, the northern hemisphere is more directly exposed to the Sun's radiation, while the southern hemisphere is not. This changes one half of a year later, when the southern hemisphere is more directly exposed to the sunlight, while the northern hemisphere is experiencing the winter season.
Earth's orbit around the Sun
The change in season at a given time during the year among hemispheres is also caused by the Earth's orbit around the Sun. The orbit of the Earth around the Sun, makes the Sun appear to move in the sky at a rate of approximately 1° every day. The sun's trajectory in the sky varies throughout the year. When the northern hemisphere is tilted towards the Sun, the days are longer and the solar radiation causes warmer temperatures. Whereas, when the tilt of the Earth points away from the Sun, the days are shorter and the radiation from the Sun is minimum, leading to colder temperatures.
The Earth receives more sunlight during the months of May, June and July. This marks the summer season in the northern hemisphere and it is when the Earth's tilt is facing more directly the Sun. The Sun reaches its highest altitude during the summer solstice, which marks the day with the longest daylight and shortest night. In the summer, the Sun is higher in the sky, producing more radiation, which produces changes in the weather and ecology of the Earth. As the Earth continues its orbit around the sun, the length of the day varies as well as its altitude.
The months of August September and October mark the Autumn season in the northern hemisphere. During the autumn season, the Sun begins to lose altitude, moving southward. During this season at the autumnal equinox, the path of the Sun crosses the equator, marking the time at which the day and night are of equal length and the Sun can be seen directly overhead at the equator. During the fall, the plants and trees begin to change in color and fall to the ground due to the lack of sunlight received by the plants to produce chlorophyll. The Earth's orbit causes the sun to appear as if moving southward, losing altitude.
During winter, in the months of November, December and January, the sun appears lower down in the sky, reaching its lowest altitude at the winter solstice. During the winter solstice, which occurs around December 21st, the sun reaches its lowest angle in the sky in the northern hemisphere. The time of the winter solstice marks the shortest day and the longest night of the year in the northern hemisphere. Because the sunlight hits the Earth in an oblique angle, the insolation received is minimum and the temperatures are low.
The months of February March and April mark the spring season in the northern hemisphere. During this season the Sun appears to be moving northward in the sky. At the vernal equinox, which occurs in March 21st, the sunlight is received equally in both hemispheres and the day and night, again, are of equal lenght. The tilt of the Earth is at a 90 degree angle with the sun, while the path of the Sun crosses the equator at the vernal equinox. During the spring season, the days become sunnier and warmer and the plants and trees begin to bloom.
The seasons experienced in one of the Earth's hemispheres are opposite to those of the other hemisphere. When in the northern hemisphere is winter, in the southern hemisphere is summer and when in the northern hemisphere is summer, in the southern hemisphere is winter. Each season of the year is followed by a change in floral and animal behavior, such as the migration of birds. According to phys.utk.edu, the seasons of the year are produced by the relative location of the sun in the sky with respect to the tilt of the Earth.