Hi Tom, > My experiments indicate that the size and brightness of a reflection of > the Sun from a shiny sphere is a function of the radius of the > reflecting sphere, and the OBSERVER's distance to the reflecting sphere, > exactly as you said. > But here is the interesting part: The phase angle DOES appear to affect > the reflection, but the wacky bit is that the reflection gets BIGGER as > the angle gets shallower. Meaning, with the sun at my back, the > reflection is a small dot near the center of the sphere as described. > Turning around, with the Sun reflecting off of the sphere, the image > spreads out into a crescent as it gets nearer the edge (shallower angle) > and gets brighter. I'm fairly certain that for an ideal specular surface illuminated by a point source (or in this case a source of comparatively small angular size -- the sun), the overall reflected irradiance is independent of phase angle. (I haven't actually tried tracing a bunch of rays recently to confirm that this is true.) There may be some small deviation due to the fact that the sun isn't a point source. What I think you are seeing when you get to shallower phase angles (and the solar reflection spreads into a crescent) is the same total number of photons/sec, but they are coming from a larger surface area of your sphere. So while the "size" of the distorted image of the sun has gone up, the brightness per unit solid angle has gone down by a corresponding amount. It might be difficult to convince yourself of this without two identical spheres at widely different phase angles that you could quickly look back and forth between in order to judge the comparative brightness of the highlights. See if you can dig up some old observational notes on the Pageos balloon (before it broke up). I think you'll find that the visual magnitude of this satellite was dependent only on range, not phase angle. Cheers, Rob ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Fri Feb 09 2001 - 14:02:44 PST