Spring!

Spring-Vernal Equinox: March 21, 2007

II Equinox and Solstice

In late March and late September both hemispheres
are the same distance from the Sun and the Sun is
said to be at an equinox. The Northern Hemisphere’s
vernal equinox usually occurs on March 20 or 21 and
marks the beginning of spring. The Northern
Hemisphere’s autumnal equinox usually occurs
on September 22 or 23 and marks the beginning
of autumn. In the Southern Hemisphere, the vernal
equinox occurs in September and the autumnal
equinox occurs in March. At the equinoxes, the Sun
appears to be directly over Earth’s equator. The
lengths of day and night are then equal over almost
all Earth, except at the poles. At the North Pole and
South Pole, Earth’s atmosphere bends the Sun’s rays
enough to make the Sun visible throughout the day
and night, even during the 12 hours the Sun is below
the horizon.

The Sun is said to be at a solstice when the
difference between the distances from each
pole to the Sun is at its greatest. The solstices
usually occur on December 21 or 22 and June
21 or 22. In December, the South Pole is tilted
farther toward the Sun than it is at any other
time of the year and the North Pole is tilted
farther away from the Sun; the Southern
Hemisphere has its summer solstice and the
Northern Hemisphere has its winter solstice.
In June, during the Southern Hemisphere’s
winter solstice and the Northern Hemisphere’s
summer solstice, the North Pole is at its most
direct tilt toward the Sun, and the South Pole
tilts away from the Sun. The hemisphere most
tilted toward the Sun on the solstice experiences
its longest day of sunlight and its shortest night.
The other hemisphere experiences its shortest
day of sunlight and its longest night.

The seasons have an unequal number of days
because Earth’s orbit is slightly elliptical, or
oval shaped, and the Sun is not exactly at the
center of the orbit. Earth moves slightly faster
when it is close to the Sun than when it is
farther away, so the seasons that occur when
Earth is close to the Sun pass more quickly.
Earth is closest to the Sun in January and
farthest away in July, so the summer is
longer than the winter in the Northern
Hemisphere. In the Southern Hemisphere,
the winter is longer than the summer.

"Season," Microsoft® Encarta® Online
Encyclopedia 2007 http://encarta.msn.com ©
1997-2007 Microsoft Corporation. All Rights
Reserved.
© 1993-2007 Microsoft Corporation. All Rights
Reserved.

Season-Encarta

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