Physics Photo of the Week

November 5, 2010

Smiley Moon and upright Moon
WWC Graduate Joel Barto, a reader of Physics Photo of the Week, recently asked the question: "When we view the crescent Moon near sunrise or sunset, why does it sometimes appear as a 'smile' and other times it appears upright, like a parenthesis?"  My first thought was: "That is a very good question - I had never thought about that before nor noticed it before!"  Thank you, Joel, for inspiring a Physics Photo.

I had to wait until 2 equinox's had passed in order to obtain pictures that Joel had suggested.  Both these photos show the crescent Moon in the pre-dawn sky.  The top photo, made this past September, near the fall equinox clearly shows the "smiley" Moon.  The bottom photo from this past April, close to the Spring equinox, shows the Moon more erect resembling a parenthesis.

The Moon and other planets all orbit the Sun in roughly the same plane - the
ecliptic plane - which is the plane of the Earth's orbit.  The ecliptic plane is tilted 23 1/2 degrees from the celestial equator.  This is another way of stating that the Earth's equator is tilted 23 1/2 degrees from its orbit.  When plotted on the sky, the ecliptic crosses the celestial equator at two points - the equinoxes.  The drawing below shows the Sun at both the fall equinox and the spring equinox.  The ecliptic is oriented about 23 degrees above the equator at the September equinox and about 23 degrees below the equator at the March equinox.  Because the Moon is close to the ecliptic throughout its orbit, the crescent Moon in the eastern sky before sunrise shows the "smiley" or upright crescent.

When the crescent Moon appears at sunset, it "smiles" at the spring equinox and is upright at the fall equinox reversing the orientations.














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.


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