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. 

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