🚀 go-pugleaf

In article <73361@cup.portal.com> Van@cup.portal.com (Sylvan Van Jacques) writes:
>  I was once told that the apparent brightness of the Sun would be the same
>from Venus, Earth, Mars, or at any distance. It would only appear smaller.
   As you get further away, of course.
>
> Is this true? How does one reconcile this with the 1/r^2 decrease in the
>intensity of radiation received by a given area, whether it be the eye or a
>planet? (Venus gets a lot more radiation than the Earth.)
>
> One could say that the reason the flux goes down as
> 1/r^2 is that the apparent area of the sun goes down as 1/r^2, but
> the apparent brightness of the sun is the same.
     Exactly.
> 
> Does this breakdown when we get so far way that we can't resolve the source?
> Point sources, like stars, do obey the 1/r^2 law for apparent brightness,
>don't they?
     Yes and yes.
>
>  How about this? Suppose that the Sun had twice the radius, but the same
>brightness. Any surface would receive 4 times as much radiation (wouldn't it?).
> But it would not appear any brighter to our eyes (again, unless it were so far
>away that we couldn't resolve the surface). Isn't this the inverse of what I 
>posted above?
    All three correct.
>
> Does this make sense to anyone?
> Has anyone else heard this about the sun?
>  Does anyone see this as an apparent paradox?
>  Any comments at all?
> Van
>  van@cup.portal.com

     The result certainly seems to be counter-intuitive, since it suprises 
most people the first time they hear it. It is not paradoxical, however, since
you get the same results no matter what path your discussion follows.

-- 
    \ _____ /       Maurice Barnhill, physicist and birdwatcher     /\___/\
     |_____|     mvb@brahms.udel.edu       University of Delaware  | O   O |
    /       \    FAF00911@udelvm.bitnet    Newark, DE 19716        |___V___|