Monday, September 25, 2017

The Atwood Sphere!

While my last post stated that Adler Planetarium is America's oldest planetarium, there is an older device for visualizing the sky, if not an actual planetarium in the usual sense! Known as the Atwood Sphere, it is a 17 foot diameter sphere made out of thin gauge (1/64 inch thick) metal, made by a windmill company in 1913. Originally installed at the Chicago Academy of Science, it was carefully drilled indicating the positions of nearly 700 stars, and when viewed from the interior, provided a reasonable approximation of the night sky from light coming in the holes. Over the years the exterior was actually painted to resemble a globe of earth with continents, oceans and even the vertical relief of mountains pounded out from the interior. It fell into disuse after Adler came into being in the 1930s with it's state-of-the-art Zeiss projectors and massive dome, but the Adler acquired it in the mid 1990s, restoring the sphere to its original design, color, and construction with a motorized drive approximating the rotation of the stars overhead. The photo at left shows the "car" that will move up to 10 friendly people up into the viewing area of the sphere interior, at right, my finger points out one of the stars drilled into the surface...

The guides do a great job with a batch of visitors moving through the Sphere every 8-10 minutes. Besides the light coming into the holes from the upper illumination, there are constellation lines put on with luminous paint, kept bright with dim UV illumination. The image at left shows a view of the interior - if you know your way around the sky, you can see the late winter northwestern sky with the "W" of Cassiopeia at center, Andromeda at left, and Perseus on top. I thought the holes-in-steel worked remarkably well - well enough evidently that during WW2 they offered lessons in celestial navigation to flyers undergoing training at local bases!

At right is the drive mechanism that rotates the sphere around the viewers in the "car" to provide a leisurely tour of the entire sky in a few minutes...

Edge of Atwood Sphere - remind you of anything?
At left is a Gif of 6 frames that I took as the guide took a new group up into the sphere. As a joke, he stated that the short ride was the "world's slowest roller coaster"! Note the sphere starting its rotation in the last frame... It is a remarkable display of the ingenuity and inspiration of Wallace Atwood, who served on the board of directors of the Academy of Science. And it is remarkable today that Adler got it out of mothballs and made it into such a popular display!

At right is a view of the upper edge of the sphere against the illumination from above that provides light for the "stars" punched in the surface. Perhaps faintly you can still see some of the vertical relief they punched into the globe, or perhaps it is evidence instead of rough handling in the past. Regardless, it reminds me of an image taken a couple years ago as New Horizons looked back at Pluto. It is such a lovely and amazing image, show me the edge of a sphere and I'll think back to this image often:

Looking at edge of a sphere will always remind
me of New Horizon's look back on Pluto...

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