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Seasons are broadly characterized by the difference in average temperature and the length of day
or night. Intuitively, it may seem that the varying distance of Earth from Sun causes
seasons. But in fact, the maximum difference of this distance is just about 3%. The real
cause of seasons is the tilt of Earth's rotational axis relative to the plane of revolution.
When the Northern Hemisphere is tilted towards the Sun, it has longer days, receives more
of the Sun's rays and the Sunlight hits more directly. This happens around June of every
year when the Northern Hemisphere has summer and Southern hemisphere has winter. Conversely,
around December, Northern hemisphere is tilted away from the sun, thus experiencing winter.
These extremes, when the poles are either farthest or closest to the Sun, are called
Solstices when we have the longest days or nights.
During late March, both the hemispheres are at equal distance from the Sun, and thus have
equal length of day or night. This is called an equinox and it marks the beginning of spring
for Northern hemisphere and autumn for Southern hemisphere. Similarly, in late September we
have another equinox heralding the other two seasons. Note that Solstices are not the positions
when Earth is closest or farthest from the Sun. In fact Earth is closest to the Sun in
January. Also, the axial tilt which is constant throughout a year, does change over long periods
of time.