Sunday, May 6, 2012

Super Moon 2012 - or Is It SuperMoon?

Via wikipedia
http://en.wikipedia.org/wiki/File:Supermoon_comparison.jpg
I have been a little tardy in getting to the subject of the "supermoon".   The United States Naval Observatory (USNO)   says that the full moon occurs on May 6, 2012 at 03:35:00 UTC.  Since Boston is currently at UTC minus 4 (Eastern Daylight Time), the full moon happened at 11:35 last night. 

Apparent Size Change


The distance change from average orbital radius to perigee (closest approach)  is about 6%.  The moon is not a point source when viewed from Earth so we see an apparent change in size  (about 6% in diameter and 12% in area).  (see image on right).   If a far away, "point source", like a star, got 6% closer to us, we would not see any size change - only a change in apparent brightness.


Apparent Brightness Change

In addition, light from each location, or point,  on the moon spreads out in all directions. Since the light from each point travels a shorter distance to earth, the famous inverse square law says that detector (eye) would receive about 12% more light,  from each point on the moon, due to the change in distance. With an imaging detector like the eye, or a camera. each point on the film or retina receives more light independent of the size of the moon.  This change is unlikely to be perceived  as a big change brightness of the moon but it may add to the overall effect of a "super moon" , or supermoon.   

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