Physics Photo of the Week

December 19, 2008

Earliest Sunset
When 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 December 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 position of 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 solstice.

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 latest sunrise.

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!

Physics Photo of the Week is 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 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.