Expert: Tom Whiting - 12/19/2008

Question
If I hold up a piece of paper with the sun behind me, it appears darker as I turn it from perpendicular.  More light per unit of area and reflecting more directly back at me should always give a brighter reflection, shouldn't it?  If I look at a ball with the sun behind me, the light reflects more directly back from the center and the ball appears brighter there than at the edges.  So why is a full moon as bright at the edges as in the middle?  Why isn't it bright at the center and darker at the edges?  (As in this photo of Jupiter.)

http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=ig328_jupit...

Answer
Hi Steve,
Your picture of Jupiter doesn't count...that's NOT a full Jupiter
as it's only about 98% illuminated.  That left hand side is partly
on the night side of Jupiter, it's not completely full.
(Otherwise, you'd see a sharp demarcation line like you do on
the right hand side of the picture). But notice Jupiter does exhibit
polar limb darkening because of it's atmosphere, both thinkness
and composition differences).

But to your question, the sun exhibits "limb darkening" because
it EMITS light (whereas the other bodies in question are just
reflecting light).  A spherical emitter like the sun has limb
darkening because the limb light is being filtered down by a deeper
edge atmosphere, as the radiation is coming from within the sun
itself.  With the moon, it's a different story, it's just reflecting
sunlight (just like a daytime scene on the Earth), and with NO atmosphere either. So it's edge characteristics will be based more upon the texture and coloration of the soil at those edges. That's why the maria (lava plains) appear darker to us than the lunar highlands.
PLUS the moon does not have an atmosphere...if it did, then we would
see some apparent limb darkening as the light would have
a thicker layer of atmosphere to filter it down, out on the edges.
Hope this explanation helps,
Clear Skies,
Tom Whiting
Erie, PA