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>Optic Illusions We Can Understand

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WHY IS THE MOON BIGGER AT THE HORIZON?

Is it an astronomical phenomenon or is your brain being duped?

By Mark Thompson 
Thu Jul 15, 2010 09:19 PM ET 
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Moonset

A moonset behind the Very Large Telescope (VLT) at the European Southern Observatory (ESO), Paranal, Chile. Is the moon the right size? Click to enlarge this image. 
G.Gillet/ESO
We’ve all been there, done it and got the t-shirt; glanced out the window and noticed a stunning, slightly orange-colored yet huge moon peeping over the horizon.
Maybe you’ve even tried to take a photograph of it only to be disappointed when confronted with a tiny image of the moon’s disk. That’s not how you remembered it… right?
You my friend have been duped, not by the strange effects of the Earth’s atmosphere but by one of nature’s very own optical illusions.
It’s fair to say at this point that the refractive properties (ability of the gas in the atmosphere to bend light) doslightly change the shape of the moon but the effects aren’t huge and it’s not this that is responsible for the “Moon Illusion.”
In fact, you can prove yourself that it’s just an illusion: take a long stick plus a small coin and when the moon is low, hold the piece of wood and attach the coin to it so the coin just covers the moon in the sky. Then wait until the moon is a lot higher later that night and check. The moon’s size hasn’t changed at all.
Lots of people have asked me over the years why the Moon is bigger when it’s nearer the horizon than when it’s high up in the sky. The answer is really quite simple and it lies in the way that the eye/brain combination interprets distance clues.
For example, take a look at this picture:
As you look at it, your brain interprets the path and brick wall heading off into the distance. Now turn your attention to the elephants. The one at the back looks bigger than the one at the front, right? Wrong, they are all identical.
The actual image that forms on the back of your eye shows each elephant is the same size. That is until your brain gets in on the act. Because you perceive the path heading off into the distance, you also assume the elephants are at different distances.
However, because the image in your eye of each elephant is identical, your brain decides that the elephant at the ‘back’ must be bigger than the elephant at the ‘front’ to allow it to form the same sized image. This illusion is named after its Italian discoverer Mario Ponzo who first demonstrated it in 1913.
Ponzo Illusion: 1 – You: 0.
So how does this apply to the moon? There are plenty of clues to distance on the horizon, other than the fact that we know the horizon is a long way away. Even clouds or flocks of birds are typically further away when near the horizon so we assume anything near the horizon is at a great distance, even the moon is perceived to be further away than when higher in the sky (due to the lack of distance clues).
Just like the elephants in the picture, the image of the moon in our eyes are identical regardless of whether it is high up or low down but the brain does the same thing again. It assumes that if both images of the moon are the same size because one is down by the horizon and thus further away, it must actually be a lot bigger to form the same size image! So our brains interpret the moon as looking bigger.
Apparently if you turn with your back to the moon bend forward and look at it through your legs, because you see everything upside down and different to usual, your distance clues are all gone and the moon looks its normal size. I’ve never tried that myself though, so I’ll leave that up to you.
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>A Swimming Bot Inside Your Body?

>he Boston Globe describes the efforts of a Japanese-born scientist to develop new technologies for use in surgery. His last project is to build a swimming robot designed to explore the human gastrointestinal tract (GI tract) from esophagus to colon. This 2-centimeter long robot will have a swimming tail to deliver the energy picked from the outside and use it to steer it in the GI tract. It will also be able to send back images to the physicians and to deliver therapy. Coincidentally, the Philadelphia Inquirer is reporting about another medical robot helper able to crawl like an inchworm into your heart. This second robot, the HeartLander, is designed by Cameron Riviere, an associate research professor at Carnegie Mellon University’s Robotics Institute in Pittsburgh. But it’s not an independen robot.

The device is inserted through a small incision in the chest and controlled with a wire tether by an external operator. Two suction-pad feet and a flexible midsection enable the device to move at about half a foot a minute. “It is not completely independent or detached, but can freely move with the beating heart,” Riviere said. Just 6 millimeters high and 8 wide, the robot can squeeze into the space between the heart and its outer lining. Tiny holes on the feet create a vacuum that ensures HeartLander remains attached.

Now, let’s move to the swimming robot designed by Nobuhiko Hata, technical director of image guided therapy at Brigham And Women’s Hospital and a professor of radiology at Harvard Medical School. Here are his comments about a trip to Japan which led him to the design of this swimming robot.

“In Japan they’re thinking seriously on integrating robots as partners” with human beings, he says. This cooperative philosophy inspired Hata to think about applying mechanical techniques to problems he had previously only been thinking about through computers. The result: “the swimming robot.”

There are similar devices under development, but Hata’s has the distinction of “swimming” — using oscillations in the magnetic field of an MRI machine to power its fins. It will literally dart like a minnow through the body’s cavities, taking images, and, perhaps, releasing chemicals or directing a tiny laser at the problem area. Hata’s robot is still in the early stages of development, but this month he will present a paper on it at a radiologists’ conference in Berlin.

In “Radiology and GI Imaging Close the Divide,” Imaging Technology News gives more details about this active swimming endoscope. (Cristen C. Bolan, Imaging Technology News, April issue 2007)

The current capsule endoscope on the market is passive — it moves down the GI tract starting with parastatic movement of the esophagus. The problem, according to Hata, is that you cannot drive the capsule toward suspicious lesions nor can you position it to do more precise scrutiny of a lesion. Hata believes that the solution to the problem is to develop an actively steerable endoscope, and, to this end, he is developing swimming micro-robots — un-tethered endoscopes for transmitting images from inside the body.

“Our technology is unique in the sense that it swims. Others have developed an endoscope, rowing it through the GI tract, but we believe that a swimming tail is the most efficient way of delivering the energy from the outside and then converting it to a propulsion pulse to steer it in the GI tract,” said Hata. The propulsion is inspired by a novel propulsion theory based on flagellar motion and is achieved by creating a traveling wave along a tail made of piezoelectric material decomposed into the natural modes of the beam. According to Dr. Hata, three individual waving tails, controlled by a magnetic field, are designed to swim in any direction.

Let’s finish with Hata’s philosophy, as reported by the Boston Globe: “On the day somebody closes my coffin, I want to count how many people I’ve saved. If it’s more than one, then my presence was worthwhile.” I could paraphrase this: if I’ve helped some readers of this blog, I would have been useful.
Sources: Andrew Rimas, The Boston Globe, May 7, 2007; Josh Goldstein, The Philadelphia Inquirer, May 7, 2007; and various websites
You’ll find related stories by following the links below.