The Equinox (Vernal & Autumnal)
There are only two times of the year when the Earth's axis is tilted
neither toward nor away from the sun, resulting in a "nearly" equal
amount of daylight and darkness at all latitudes. These events are
referred to as Equinoxes.The word equinox is derived from two Latin
words - aequus (equal) and nox (night). At the equator, the sun is
directly overhead at noon on these two equinoxes. The "nearly" equal
hours of day and night is due to refraction of sunlight or a bending
of the lights rays that causes the sun to appear above the horizon
when the actual position of the sun is below the horizon.
Additionally, the days become a little longer at the higher
latitudes (those at a distance from the equator) because it takes
the sun longer to rise and set. Therefore, on the equinox and for
several days before and after the equinox, the length of day will
range from about 12 hours and six and one-half minutes at the
equator, to 12 hours and 8 minutes at 30 degrees latitude, to 12
hours and 16 minutes at 60 degrees latitude.
The Solstices (Summer & Winter)
The summer solstice occurs at the moment the earth's tilt toward
from the sun is at a maximum. Therefore, on the day of the summer
solstice, the sun appears at its highest elevation with a noontime
position that changes very little for several days before and after
the summer solstice. The summer solstice occurs when the sun is
directly over the Tropic of Cancer, which is located at 23.5°
latitude north, and runs through Mexico, the Bahamas, Egypt, Saudi
Arabia, India, and southern China. For every place north of the
Tropic of Cancer, the sun is at its highest point in the sky and
this is the longest day of the year.
The winter solstice marks the shortest day and longest night of the
year. In the Northern Hemisphere, it occurs when the sun is directly
over the Tropic of Capricorn, which is located at 23.5° south of the
equator and runs through Australia, Chile, southern Brazil, and
northern South Africa.
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We all know that the Earth makes a complete revolution around
the sun once every 365 days, following an orbit that is
elliptical in shape. This means that the distance between the
Earth and Sun, which is 93 million miles on average, varies
throughout the year. During the first week in January, the Earth
is about 1.6 million miles closer to the sun. This is referred
to as the perihelion. The aphelion, or the point at which the
Earth is about 1.6 million miles farther away from the sun,
occurs during the first week in July. This fact may sound
counter to what we know about seasons in the Northern
Hemisphere, but actually the difference is not significant in
terms of climate and is NOT the reason why we have seasons.
Seasons are caused by the fact that the Earth is tilted on its
axis by 23.5°. The tilt's orientation with respect to space does
not change during the year; thus, the Northern Hemisphere is
tilted toward the sun in June and away from the sun in December,
as illustrated in the graphic above.
For a complete listing of the dates of the winter and summer
solstice's and spring and fall equinox's through 2020, check out
this site from the
U.S. Naval Observatory.
[Text copied; NATIONAL WEATHER
SERVICE CENTRAL ILLINOIS WEATHER FORECAST OFFICE, LINCOLN]