Re: Private Universe and the Seasons

[Larry Woolf <larry.woolf@GAT.COM>]

Just to clarify some issues that Richard Hake attributed to me, my actual
comments are shown below in quotes:

Note that I also mention the need to clarify what is meant by a ray.  Also,
I never stated that the rays from the sun were directed radially outward.
The model that shows the sun's rays being directed radially outward is of
course not true -every point on the sun's surface effectively emits light in
all outward directions - as do all extended sources.

----------------------------------------

"In addition, few texts address any of the following issues:
1. why are the sun's rays parallel at the earth
2. how do the sun's rays look when they leave the sun
3. why the sun's rays are shown in a radial fashion leaving the sun, then
are parallel at the earth; and why the radial diagram is also a
simplification
4. why the sun's rays are not really parallel at the earth, but that the
parallel nature is a useful simplification
5. what is meant by a ray of the sun - what is it a model for?
6. why the earth doesn't just keep getting hotter because it is heated by
the sun - cooling mechanisms are never discussed so energy conservation is
generally put on hold when discussing the seasons."

and

"Consider one of the examples
that I used earlier:

<http://hea-www.harvard.edu/ECT/the_book/Chap2/Chapter2.html#oots>
where the first figure encountered shows us that the sun emits all parallel
rays out the 2 sides of the sun.

Suppose you are using this model to help students understand that rays that
the sun emits are parallel at the surface of the earth and this concept is
needed to help understand the seasons. Fine.  But what happens if you then
probe and ask about the light rays that are emitted by extended objects like
the sun, light bulbs, etc? Then the misleading model you used in discussing
the seasons leads to a major pre-existing misconception about light emission
that you have fostered. How will you then bring up light from extended
objects, radial light emission from a point source, the parallel nature of
nearby rays at far distances from the light source?  Where is the useful
model that shows light rays from the sun with the light rays that end up at
the earth all nearly parallel.  I haven't seen it yet.  That, to me, seems a
useful model to keep in mind."

Larry Woolf
General Atomics
3550 General Atomics Court
Mail Stop 78-110
San Diego CA 92121
Ph:858-526-8575
FAX:858-526-8568
http://www.ga.com
http://www.sci-ed-ga.org


> -----Original Message-----
> From: Jim Green
> Sent: Friday, September 19, 2003 9:07 AM
> Subject: Re: Private Universe and the Seasons
>
>
> > > ///The Sun's rays leave the Sun nearly radially but are nearly
> > > parallel to one another when they reach the Earth [as stressed by
> > > Larry Woolf (2003a,b)].
>
> In the midst of this discussion could someone please tell me what a "Sun
> ray" is.  Is it rerally helpful to talk about such things?