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[Phys-L] Re: In the Private Universe



I was about to reply before reading to Vickie's end, so I'll just add
how I didi it.

I "won" a teaching job because I said one could do better w/ a candle
and a large apple and a rather small one to show eclipses than drawings
on the board, etc. For the seasons insert kebob stick thru the apple,
etc. I've tried a slide projector and found candles much superior. A
mini-mag lite w/ the reflector removed is a good candle.

bc

Frohne, Vickie wrote:

Having taught astronomy myself, I think that the root source of the m=
isconceptions regarding the explanation of the seasons is that the ge=
ometry is inherently difficult to understand. First of all, illustrat=
ions in books can NOT depict the scale of the solar system correctly =
if the planets are to be visible on the page. Therefore, illustratio=
ns showing the tilt of the Earth toward the Sun invariably give the i=
mpression that the northern hemishphere is considerably closer to the=
Sun in the summer. In the pictures, it IS. This notion goes away o=
nly when one realizes that the "closer" is insignificant compared to =
the Earth-Sun distance. Secondly, students have a great deal of diff=
iculty deciphering the illustrations. When the Earth's orbit is pict=
ured as an elongated ellipse on the page, students interpret it as an=
ellipse. I can explain that this is a circle viewed from an angle, =
but later we go talk about comets, whose orbits really *are* that elo=
ngated, and drawn as such. So - Earth: circular orbit drawn from the=
side. Comet: elliptical orbit drawn from the top. Same picture! Wh=
at do you expect the students to think??? I don't blame them a bit f=
or being confoozled. It took me years of thinking about this to get i=
t right, too! =20

To understand the seasons, students absolutely need to see it in thre=
e dimensions. I strongly recommend putting globe pencil sharpeners a=
nd flashlights into their hands. A demo won't work - they have to man=
ipulate the items themselves, while being told what to look for. The=
students need to observe the shadows on the globes as they move the =
globe around the light, keeping the axis always pointed to a particul=
ar corner of the ceiling. Once they do this exercise, they'll have a =
fighting chance of being able to interpret the pictures in their book=
s. Computer animations help, too, but they suffer from most of the s=
hortcomings of the book illustrations. (I once reviewed the computer=
animations for a popular college-level astronomy text - they were al=
l wildly inaccurate in some way. One showed the Earth rotating with =
its axis coplanar to the Moon's orbit - eeeewwww.)

By the way, as long as you have the globes & flashlights out, you can=
address the phases of the Moon at the same time.

Globe pencil sharpeners are small compared to the beam of light, so t=
hey show the effect better than a larger globe. They also have gimbal=
mounts. If you search your dollar stores, sometimes you can even fi=
nd globe pencil sharpeners that have a correct map scale on 'em. The=
flashlight "suns" have to be rotated in place to face the earth, but=
they have the advantage of not flooding the whole room with light. =
Turning the flashlights doesn't seem to bother the students any - the=
y know what the Sun looks like, and understand the symbolism. If you=
want larger globes, one can sometimes find 10" inflatable globes, wi=
th gimbal mounts, in dollar stores. I've found lots of good physics =
stuff in dollar stores - globes, magnets, flashlights, balls, etc. T=
he cheap cellophane tape is great between crossed polarizers. Caveat=
: If you do find good stuff in dollar stores, buy your class set imme=
diately because you can't depend on it being there tomorrow. Advanta=
ge: $, not $$$$

Vickie Frohne


-----Original Message-----
=46rom: Forum for Physics Educators on behalf of Joseph Bellina
Sent: Wed 2/23/2005 7:24 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: In the Private Universe
=20
I agree completely. It is interesting to see what you get when you a=
sk
people which of three pictures is the best illustration of the earth'=
s
orbit around the sun. The near circle, an elongated ellipse with the
sun at a focus or an elongated ellipse with the sun in the center.
Almost no one picks the near circle.

joe