The Seasons and
The Earth's seasons are not caused by the
differences in the distance from the Sun throughout the year (these
differences are extremely small). The seasons are the result of the
tilt of the Earth's axis.
The Earth's axis is tilted from perpendicular to the
plane of the ecliptic by 23.45°. This tilting is what gives us the
four seasons of the year - spring, summer, autumn (fall) and winter.
Since the axis is tilted, different parts of the globe are oriented
towards the Sun at different times of the year.
Summer is warmer than winter (in each hemisphere)
because the Sun's rays hit the Earth at a more direct angle during
summer than during winter and also because the days are much longer
than the nights during the summer. During the winter, the Sun's rays
hit the Earth at an extreme angle, and the days are very short.
These effects are due to the tilt of the Earth's axis.
The solstices are days when the
Sun reaches its farthest northern and southern declinations. The
winter solstice occurs on December 21 or 22 and marks the beginning
of winter (this is the shortest day of the year). The summer
solstice occurs on June 21 and marks the beginning of summer (this
is the longest day of the year).
are days in which day and night are of equal duration. The two
yearly equinoxes occur when the
Sun crosses the celestial equator.
The vernal equinox occurs in late March (this is the
beginning of spring in the Northern Hemisphere and the beginning of
fall in the Southern Hemisphere); the autumnal equinox occurs in
late September (this is the beginning of fall in the Northern
Hemisphere and the beginning of spring in the Southern Hemisphere).