MadSci Network: Physics |
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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