From: pauld@exploratorium.edu
Date: Thu Jun 13 2002 - 10:20:59 PDT
Message-Id: <200206131720.g5DHKeY00334@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole Eclipse viewing with pegboard Date: Thu, 13 Jun 2002 10:20:40 US/Pacific
Hi George
The pegboard was one of my favorite ways to project the eclipse too.
The fuzziness of the image wasn't due to the penumbra. It was due to the large
size of the "pinhole" in the pegboard. (Although it has a similar origin to the
penumbra it isn't called that.)
Smaller pinholes lead to sharper, dimmer images.
You can do a neat experiment with a variable aperture shutter scavenged from one
of the many dead lenses in the free camera collection at the Explo.
Start with a wide open aperture and project a pinhole image of the sun, then make
the hole smaller and smaller, watch as the fuzziness decreases.
With a crescent sun, the place where the crescent is thickest makes for a
brighter spread of light and so you see the fuzziness better in some places
around the image than in others.
One other thing, during maximum partial eclipse the sun becomes a non-round
source of light, almost like a line source.
If you cast the shadow of a finger, the edges of the finger parallel to the line
of the sun have sharp edges to their shadows while the edges of the shadow of the
sides of the finger perpendicular to the line of the sun have fuzzy shadows.
It was fun to stick out a finger and rotate it and watch the shadow edges get
sharp and then fuzzy. (These fuzzty edges of shadows are indeed penumbras while
the dark inner shadow is an umbra.)
Paul D
The next partial eclipse visible from San Francisco will be in 2012. It will be
an annular eclipse in northern California.
> Pegboard was great. I grabbed a sheet from a staffer at LHS Monday and just
> stood there casting dozens of images on the deck.
> Interaction with kids, small and big was constant.
> However Chron reporter Charles Buress (I assume from the quote in Tewsday's
> Chron) asked me about the fuzziness of our shadows. Was it due to the
> eclipse? He said 'penumbra' in describing it. Shadows looked kind of fuzzy
> on one side.
> I said "penumbra sounds good, but I want to ask Paul."
> 'Paul who?'
> "The famous Paul Dougherty of the Exploratorium; we communicate by internet
> news group."
>
> So, Paul, my optics is rusty by now.
> I took a sphere, cast a shadow, and snapped the image. Looks fuzzy one one
> side. I remember that spheres on flagpoles are to cast clear shadows for the
> annalemma.
> I know I was standing in the penumbra. Was there anything? Or was it my
> ancient eyes?
>
> George Fosselius
> This year C++ programming at Albany High, Keyboarding at Albany Middle
> This Summer: Project QuarkNet at LBNL
> Next year--??
>
>
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