In the United States and
the rest of the northern
hemisphere, the first
day of the winter season
is the day of the year
when the Sun is farthest
south (on December 21st
or 22nd).
This day is known as the
Winter Solstice.
The declination of
the Sun on the winter
solstice is known as the
tropic of capricorn
(-23° 27'). In the
Southern hemisphere,
winter and
summer
solstices are exchanged
so that the winter
solstice is the day on
which the Sun is
farthest north.
A common
misconception is that
the earth is further
from the sun in winter
than in
summer.
Actually, the Earth is
closest to the sun in
December which is winter
in the Northern
hemisphere.
As the Earth travels
around the Sun in its
orbit, the north-south
position of the Sun
changes over the course
of the year because of the
changing orientation of
the Earth's tilted
rotation axes. The dates
of maximum tilt of the
Earth's equator
correspond to the
Summer
Solstice and Winter
Solstice, and the dates
of zero tilt to the
Vernal Equinox and
Autumnal Equinox.

The reason for these
changes has to do with
the Earth's yearly trip
around the sun. For part
of the year the Earth's
North Pole points away
from the sun and part of
the time toward it. This
is what causes our
seasons. When the North
Pole points toward the
sun, the sun's rays hit
the northern half of the
world more directly.
That means it is warmer
and we have
summer.
The day of the winter
solstice is the shortest
day of the year. The
length of time elapsed
between sunrise and
sunset on this day is a
minimum for the year. In
the United States, there
are only about 9½ hours
of daylight on this day.
At latitude 40
degrees north, the
earliest sunset of the
year occurs around
December 8, and latest
sunrise occurs around
January 5. The day with
the least amount of
daylight (the first day
of winter), occurs
around December 21. Why
are all these dates not
the same? For a detailed
explanation, see
The Dark Days of Winter
at the U.S. Naval
Observatory web site.
Links
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Ready
- Ready.gov is a national public service
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disasters. The goal of the campaign is to get
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nation.
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The Seasons and the Earth's Orbit
- Through the Naval Oceanography Portal, the
United States Naval Observatory provides
astronomical information on the Earth's orbit
around the Sun and how it relates to the
changing seasons. |
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The Dark Days of Winter
(The United States Naval Observatory) |
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Sunrise and Sunset Times Near the Solstice
(The United States Naval Observatory) |
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Sponsored Links
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