Physics Photo of the Week
October 5, 2012
Oblate Sun
As
the Sun was setting on the evening of June 5, 2012, Venus
was still in front of the Sun. In addition to the
extremely rare occurrence of the Venus transit, the Sun has
been distorted out of round by the density gradient in the
Earth's atmosphere near the distant horizon. Viewed
from the high elevation of the Craggy Visitors' Center on
the Blue Ridge Parkway near Warren Wilson College the
optical path length to the lower part of the Sun through the
Earth's atmosphere is considerably longer than the optical
distance to the top of the Sun. The atmosphere around
the curve of the Earth resembles a lens that is thicker on
the bottom than on the top, hence it distorts the Sun to an
oval shape.
Compare this image to the earlier image taken when the
Sun was considerably higher in the sky shown in the
animation. When the Sun is high in the Sky the
difference in optical distance between the upper and lower
limb of the Sun is negligible.
Notice also that the Sun is considerably reddened as it
is setting. This very common effect is also caused by
the longer path length of light through the Earth's
atmosphere. The blue colors of the Sun's light are
scattered away from the line of sight leaving the red rays
to reach the observer.
The Venus transit of June 5, 2012 was featured in a
recent Physics Photo of the Week (September
7, 2012)
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.
Click here to see the Physics Photo of the Week Archive.
