Re: Democratic candidates address the issue of global warming while Republicans snooze...

On Fri, 04 Jan 2008 18:24:22 -0800, Islander <nospam@xxxxxxxxxxx>
wrote:

<snip>


OK, I think that the two of you are talking about different things. My
response to Jean had to do only with the change in the tilt of the
earth's axis, not the difference in the distance of the earth from the
sun.

The change in the tilt is a maximum of only 3 degrees and that occurs
over a period of about 41K years. The specific cycles that Jean
referred to happen over shorter cycles, but are still pretty long.

The change of distance between the earth and the sun is only about 4
million miles (95M at aphelion and 91M at perihelion. But, don't forget
that the intensity of the sun's rays varies as a cube law (space is 3D).
So, the difference in intensity between perihelion and aphelion as a
percentage is (95M**3 - 91M**3)/95M**3 = 13%


Hmm. That's actually only as the square, isn't it? not the cube.
It's true that the volume of a sphere increases by the cube of the
diameter, but when we receive light rays, we're not receiving them
from the volume, but only from what portion of the spherical shell
containing the light emitted from the sun at a given instant in the
past falls on us or our measuring device. The surface area of
such a shell increases by the square of the diameter, not the cube.
(Disregarding any absorption by interstellar gas, of course.)

http://tinyurl.com/25dc9q

That correlates (and has to correlate) with the fact that the
apparent size of the sun in the sky diminishes by the square
of the distance we are from it. The brightness per apparent
unit area doesn't change (disregarding absorption), but the
net luminosity received diminishes in proportion to the
diminishing apparent size, which is the square of the distance.

The brightness of a distant star identical to the sun is the
same (disregarding absorption) as the brightness we receive
from an "average" piece of the sun exactly the same area
as the apparent size of the distant star (if we could make out
the diameter of the star, which we usually can't).

<snip>



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