Re: [Phys-l] Teaching elementary astronomy topics

["John Clement" <clement@hal-pc.org>]

The terms direct and indirect are to be avoided like the plague.  Watch the
"Private Universe" video which can be found on the web.  Students take the
term direct to mean rays which come directly from the sun, and indirect to
mean reflected off something.  Actually they are using a very good
definition when they think this.  Indirect is a very bad term where
alternate meanings take precedence.

The better way of looking at it is to use the term oblique or even better
yet straight on or at an angle.  One can readily show that the illumination
of a surface changes with angle.  Then you can ask which surface will heat
up faster.  Then you can show a picture of the Earth with the rays
diagrammed and ask which places will be heating faster.  At that point you
can have various portions of the globe indicated by A, B, C... and have the
students do a ranking of which places are heating faster.  A ranking makes
them think much more than just asking for a response.

Then finally you can show a globe in the summer, winter, spring, fall
positions.  Preferably this should be an animation showing it spinning with
the sun's rays diagrammed.  Then have the students do a ranking as to the
average temperature for their location on the globe.

Be aware that many students will have severe difficulties because of
perceptual problems.  Many of them can't visualize things in 3 dimensions.
Indeed many students can't recognize shapes and match them up on maps.  The
dyslexics however should be able to visualize the seasonal effect in 3
dimensions, but might have more difficulty with flat presentations.  When
you tell them about this type of thing, those with perception problems will
always say OK, and never reveal these problems.  So getting them to give you
the answers is by far the better strategy.  That way you find out what is
going on in their heads, and can possibly make some corrections.  So with
the initial presentation of a "normal" vs "oblique" surface they should be
asked to vote on which one is heating more rapidly.  You can even have black
surfaces and have a student touch them to see if the prediction is correct.
An overhead might be used, but a heat lamp would probably be the better
option.  Come to think of it 3 surfaces normal, oblique, and parallel to the
rays would be ideal with a voted on ranking for maximum, and minimum
heating.  You could introduce the term "normal", but don't push it.  Have
the students come up with terms for each presentation, then introduce more
scientific terms, but use both for a while.  The term normal should be
introduced in other contexts to they would eventually relate that the
"normal force" means it is perpendicular.

If I had a text which used the term indirect, I would have the students
cross it out and write over it oblique or at an angle.  If it works for
Robin Williams it should work for us!

John M. Clement
Houston, TX


> > I suggest that teaching a student about "direct" and
> > "indirect" insolation perhaps confuses the student
> > unnecessarily. Talking about the seasonal variation in
> > duration of insolation is probably easier to sell, and it is
> > more important at most latitudes than angle of incidence of
> > insolation.
>
> Since the seasonal variation has to do with how long the sun is up each
> day and how directly it shines on your part of the world, I think it is
> better to teach both.  I haven't taught astronomy in a decade but I used
> to hold a cardboard cutout of the state in which my school is located in
> front of a screen illuminated by an overhead projector.  I'd tilt the
> cutout at angles corresponding to the orientation of the surface of the
> state relative to a line from the sun to the state, and the size of the
> shadow on the screen was indicative of the solar power being received by
> the state at local noon.