Physics Photo of the Week
is the earliest sunset? Contrary to what one might expect, the
earliest sunset is not on the
shortest day (longest night) of the year. The earliest sunset
actually comes about 2 weeks before the winter solstice. This is
documented in the sunset photo taken on Dec. 7, 2008, at 5:07 pm. where
the setting Sun is just visible in the center of the photograph.
The photo was taken from the landscaping rock in front of the Hamill
Science Center at Warren Wilson College.
It had been planned to obtain a photo of the setting Sun this week very
close to the December 21 solstice, however, cloudy weather all week has
prevented a photo before the deadline for writing the photo page.
A comparison photo will have to wait.
What is the reason for the earliest nights to come in early
rather than the solstice at Dec. 21? One would expect that the
date with the shortest daylight (solstice) will be the date when
daylight ends the earliest. The answer lies in the geometry of
Earth's orbit around the Sun and the fact that the Earth's orbit is
elliptical and not circular. The Earth (as well as all the other
planets) orbits the Sun in an elliptical path, not a perfect circle as
shown in the exaggerated drawing below.
It happens that the Earth is closest to the Sun in the winter and
furthest from the Sun in the summer. The Earth travels with a
higher speed when closer to the Sun (this law was discovered by
Johannes Kepler and confirmed by Isaac Newton as a result of angular
momentum conservation). The position of the Sun among the fixed
stars in the sky is a result of the Earth's orbit around the Sun.
When the Earth is traveling with greater speed closer to the Sun, the
apparent daily motion of the Sun among the stars is greater than when
the Earth is further away from the Sun. The result of all this
geometry is that the apparent Sun position advances ahead of the
its average rate when the Earth is close to the Sun - in winter.
At other times the Sun's position lags behind the average
position. This cycle
repeats in a regular fashion and is called the Equation of Time. If the
Earth's orbit were a perfect circle, the earliest sunset would occur on
The opposite effect is manifest in the time of latest sunrise in the
winter. The latest sunrise occurs about two weeks after the
winter solstice on about January 4. Perhaps we'll have a series
of comparison photos after classes resume in January comparing the
This is the last Physics Photo of the Week for 2008 as Warren Wilson
College goes on winter break until January 19, 2009. The next
Physics Photo will be published on Friday, Jan. 23, 2009 when student
work will be featured. Have a Merry Christmas and Happy Holidays!
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an 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 firstname.lastname@example.org.
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.
here to see the Physics Photo
the Week Archive.
Observers are invited to submit
digital photos to: