MadSci Network: Physics
Query:

Re: What is a Negative Shadow?

Date: Thu Jul 22 00:27:37 1999
Posted By: Troy Goodson, Staff, Spacecraft Navigation, Jet Propulsion Laboratory
Area of science: Physics
ID: 931615660.Ph
Message:

Let's look at the case of the Moon eclipsing the Sun. "If an eclipse occurs 
while the Moon is on the far side of its orbit, the Moon appears smaller 
than the Sun and can't completely cover it. Looking down from space, we 
would see that the Moon's umbral shadow is not long enough to reach Earth.   
Instead, the 'antumbral' or negative shadow reaches Earth."

the above quote and and some nice diagrams are at http://
sunearth.gsfc.nasa.gov/eclipse/SEnovice.html
In fact, I've copied one of the diagrams for this answer http://
sunearth.gsfc.nasa.gov/eclipse/image/SEDiagram3.gif

This negative shadow is the region where light that has diffused around the 
Moon overlaps and illuminates Earth.  You can get a feel for this effect by 
holding a disk (large coin or a lid from a jar) under a lamp.  As you raise 
or lower the disk, you'll notice that the amount of light that diffuses 
around the disk changes.  When you get a small, lit circle in the middle of 
the disk's shadow, you have seen a negative shadow.

I suspect you may have some questions about diffusion, so I'll direct you 
to these web pages (or, of course, any physics or optics book in your local 
library)
 
http://www.owu.edu/~mggrote/pp/physics/physics.html
 
http://en.wikipedia.org/wiki/Optics

Troy http://surf.to/tdg/



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