Path: utzoo!utgpu!watserv1!watmath!att!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!fournier
From: fournier@cs.ubc.ca (Alain Fournier)
Newsgroups: comp.graphics
Subject: Re: Lambert's Law & the Moon
Message-ID: <10506@ubc-cs.UUCP>
Date: 19 Nov 90 04:40:03 GMT
References: <27331@cs.yale.edu>
Sender: news@cs.ubc.ca
Organization: UBC Department of Computer Science, Vancouver, B.C., Canada
Lines: 26

In article <27331@cs.yale.edu> musgrave-forest@CS.YALE.EDU (F. Ken Musgrave) writes:
>...
>  Am I correct in a vague memory I seem to have, that the (Earth's) moon is
>supposedly a near-ideal Lambertian reflector?
>
>...

As this is one of my favourite trick question about reflection, I'll bite.
Consider the moon when full (that is the sun -the light source- is in the same
direction as the observer -the eye. It is pretty obvious that it appears
essentially as a disk, that is the reflected light is about of same
luminance all over the visible part of the moon (I ignore the effect
of the surface details such as the maria). That shows that it is neither
a specular reflector (the centre would be a lot brighter than the periphery,
nor a diffuse reflector (the centre would be somewhat brighter than the 
periphery (try this on your favourite renderer, a perfectly diffuse white
sphere illuminated from the direction of the eye). So what's going on.
Well, as most of the computer graphics types (us) ignore, the world is not all
between totally diffuse and totally specular, there are surfaces outside
of this. In the case of the moon, it so happens that a Phong model
(using the expression loosely) with an exponent of 0.5  for the cosine
of the angle normal/light gives the right appearance of a disk at full moon.
I can find exact references back in my office, if anybody is interested.
Credit where credit is due: Bob Woodham, of UBC, first pointed that out to me,
and has worked on the subject of models for the surface reflectance of the
moon (and other objects in the solar system).