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Re: Private Universe and the Seasons



At 10:07 -0600 9/19/03, Jim Green wrote:

In the midst of this discussion could someone please tell me what a "Sun
ray" is. Is it really helpful to talk about such things?

In the final analysis, it boils down to a photon, doesn't it?

I've been thinking about Brian's comment:

I wonder what the evidence is for a preponderance of solar
emission in the radial direction? I don't see that much
darkening of the disk at the margin....

I think he has a point. If the "rays" really do amount to photons,
they are emitted without directional preference by the atoms in the
solar photosphere. Some will follow "radial" paths with respect to
the center of the sun, but most won't. But the net effect is that
about the same number of photons leave the sun in any direction, so
that when we are a great distance from the sun, all the photons we
receive are coming in essentially the same direction, and that is
*radial* relative to the center of the sun.

The reason that the sun seems so well-defined is because the
photosphere is very thin compared to the radius of the sun, so there
is very little angular distance as seen from the earth where the
visible light from the sun goes from a maximum to essentially zero.
But since those photons that get to the earth from the limb have
passed through more of the solar atmosphere than those coming from
the center of the sun's face, there is some limb darkening, just not
very much. If the photosphere were thicker, there would be more limb
darkening, and the edge of the solar disc would seem fuzzy.

But I think his point was well-taken. The reason that we can think of
the light rays from the sun as being "radial" is that at our distance
from the sun, all the photons that reach the earth left the sun
heading is very nearly the same direction, and that is parallel to
the line joining the centers of the sun and the earth--*radial.* If
our distance from the photosphere was small compared to the solar
radius, we would see light arriving from directions covering a
significant fraction of the hemisphere facing the sun, and would no
longer talk about the rays being radial.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

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