Physics Photo of the Week
December
11, 2009
Winter Solstice and Sunset Time
The Winter
Solstice, usually December 21, occurs when the Sun is the furthest
south in the sky. If one photographs the sunset from the northern
hemisphere one can see that the position of the setting sun is furthest
south on the solstice. After the solstice the Sun sets further
and further north.
Last year I obtained a series of sunset pictures between Dec. 7, 2008
and Jan. 8, 2009 on four clear days spanning the four weeks.
Those images are displayed as an animation sequence below. The
calendar date is displaye
d as
a graphic at the top of the images. Notice that the Sun sets
furthest to the south or left in the image on Dec. 21. At other
dates it is further to the right or less south. The winter
solstice is the date at which the Sun is the furthest south in
the sky. This is due to the Earth's axis pointing away from the
Sun during the
Earth's orbit about the Sun.
The winter solstice is also the day of the year when the
nights are the longest as experienced in the northern hemisphere.
Even though the nights are the longest during the winter solstice, the
date of earliest sunset, however does not
occur on the winter
solstice, but on a date 2 weeks earlier when observed from the
mid-northern
latitudes. The bottom graphic in the animated sequence
displays the time of sunset observed on the days. Look carefully
at the Sun's position and the lower time bar. The earliest
sunset of the series occurs at 5:08 pm EST on Dec. 7. During the
solstice (Dec. 21) when the
Sun was furthest south, the sunset occured 3 minutes later at 5:11
pm. On Jan 1, the Sun's position is about where it was on Dec. 7,
but the time of sunset is 11 minutes later at 5:19 pm. The
earliest sunset and the onset of night occured this past weekend.
Now in the final 2 weeks of the fall semester, the late afternoons now
begin to become brighter, even before the winter solstice, as the Sun
has begun its cycle of later setting times. Enjoy the sunsets
this weekend!
The subtle effect on the times of sunset is attributed to the fact that
the Earth's
orbit around the Sun is an ellipse - not a perfect circle. The
Earth is closest
to the Sun in December and January. Because the Earth is closer
to the Sun at this season, it travels further in its orbit around the
Sun each day. See
the PPOW
for
December
19, 2008.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
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