Physics Photo of the Week

March 25, 2011

Equinox Sun Rise

On the Equinox the Sun rises in the due east and follows the celestial equator throughout the day.  This photo, taken on March 21, 2011 (the day after the spring equinox when we had clear weather) shows the Sun at sunrise (7:47 am Eastern Daylight Time) plus a time series of filtered Sun images taken at 20 second intervals and added together to simulate a time exposure that lasted until about 9:23 am EDT.  The much shorter sunstreak on at the right was taken on Feb. 11, 2011 and was stopped at the same time of day (9:23 if we had DST year round).  If we had a time exposure of the Sun on the winter solstice, the streak would be even further to the right and considerably shorter still.  This sequence shows the dramatic change in day length as a result of the seasons.

The equinox Sun path in the sky follows the celestial equator: the part of the sky that lies directly above the Earth's equator.  At our latitude of 35 degrees, the celestial equator is tilted 35 degrees from the vertical.  The winter Sun's path lies south of and parallel to the celestial equator.  If we were viewing the sunrises from the Earth's equator, the paths would be vertical.

To image the Sun, a special filter was placed over the camera lens to prevent over-exposure.  The filter is called a "Neutral Density 5" that is the light is attennuated by a factor of 105.

These images are part of a long-term project that will last at least a year (4 seasons).  Stay tuned...

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 

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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