🚀 go-pugleaf

In article <73361@cup.portal.com>, Van@cup.portal.com (Sylvan Van Jacques) writes:
|>  I posted this in the Space and Science section of Genie, and was met with 
|> disbelief. The more I think about this, the more it seems to make sense. I 
|> would appreciate any comments.
|>    ======
|>   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.
|> 
|>  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.)

An easy way of thinking about it: If the sun were twice as far away,
you receive 1/4 the radiation from it. But the image on your soon-to-burn-
out retina would be half the diameter, 1/4 the area, so the radiation is
concentrated by exactly the right amount for constant apparent brightness.

|>  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.
|>  
|>  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. Since the image can't get any smaller, its brightness has to change.

|>   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?

Different cases. The apparent brightness of an object is the brightness
of the image you see. The illumination provided is altogether different;
a sun twice as far away would provide only 1/4 the illumination.

|>  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

Do the explanations make sense?

Cheers,

	Bob.
-- 
bob_cannard@mentorg.com			"More dynamite!" - Theron Marks