Re: Iridium brightness compared

Ed Cannon (
Tue, 13 Jan 1998 03:39:43 -0600

Rob replied to my message comparing the brightness of an Iridium 
flare with the brightness of the not-too-far-away Moon.

> Iridium brightness compared
> 12 Jan 1998 20:47:29 -0800

This is to express much appreciation for the explanation of the 
radiance versus irradiance of an Iridium flare versus the Moon.  
Rob, thank you very much!  

For the flare I wrote about, the Sun was at -6 degrees, so it was 
twilight as well as a poor location (UT Austin campus -- plus 75% 
Moon as I found out later -- although I guess moonlight isn't a big 
deal in twilight).  So anyway my middle-aged, non-dark-adapted eyes 
probably saw it as, what, maybe about 20-50 times as intense as the 
Moon...?  It sure made me wonder what the maximum theoretical 
magnitude of those things is.  The brightest prediction I've seen 
from Iridflar is -8.  It was high up (about 67 degrees), so 
extinction was not much of a factor.

If I have one more question, it's what makes very bright flares -- 
from HST and a few others on rare occasions as well as the Iridiums 
-- appear to be extended light sources.  My guess is that it's 
atmospheric refraction of the intense light and/or eye lens 
refraction and/or something about the way the light impacts the 
retina (which likely is not held still, for example)....  Or is it 
just my eyes?  Anyway, I suppose this also is involved to some 
extent in perceived brightness.  That flare's appearance to me was 
as if it had been a significant fraction of the diameter of the 

Ed Cannon
Austin, Texas, USA