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July 2001
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Pat Murphy & Paul Doherty
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by Pat Murphy & Paul Doherty

Explore Your World

We sometimes describe the Exploratorium, the museum where we work, as a museum of science, art, and human perception.

Frank Oppenheimer, the museum's founder, pointed out that human perception is the starting place for both art and science. In an article titled "Aesthetics and the Right Answer," he wrote: "Art and science are very different, but they both spring from cultivated perceptual sensitivity. They both rest on a base of acute pattern recognition. At the simplest level, artists and scientists alike make it possible for people to appreciate patterns which they were either unable to distinguish, or which they had learned to ignore in order to cope with the complexity of their daily lives. . . . One can see only a bland flesh color in faces until a Rouault makes one aware of the violent blues and reds and purples that actually appear. Similarly, one can observe the planets rise and set without becoming aware, as Kepler did, that they are moving in ellipses about the sun."

As writers, scientists, and Exploratorium employees, we have learned to be experts at noticing things that most people are inclined to ignore. This is, ultimately, one of the best parts of the job. In this column, we thought we'd call your attention some of the natural phenomena that we love to point out to those around us. We hope that you will enjoy looking for them in the world around you.

Images of the sun

Have you ever noticed that the shade beneath a leafy tree is dappled with round spots of sunlight? The gaps between the leaves aren't round; they're all sorts of irregular shapes. But the light that shines through those irregular gaps makes round spots.

Many years ago, artist and former Exploratorium exhibit builder Bob Miller pointed out to us that those spots of light are images of the sun. The gaps between the leaves are acting like the pinhole in a pinhole camera, focusing an image of the light source—the sun.

Beginning with this observation, Bob Miller developed a series of experiments to investigate this phenomena. You can find them on line at

Or you can simply admire the dappled ground beneath a tree and know that you are standing beneath a natural, leafy, pinhole camera.

Shadows at Night

While working on The Science Explorer—Out and About, a book of science activities for kids, Pat became obsessed with shadows. She watched shadows by day, becoming fascinated with their movement. That obsession led to a year-long project tracking the sun's movement, documented in our column, "The Shadow Knows" (xxx, 1999). Pat also became aware of the shadows that follow her on any night time stroll through her urban San Francisco neighborhood.

On any urban street corner, odds are you'll cast a collection of shadows--one for each bright light source in your immediate vicinity. On one corner near her house, there are three streetlights and Pat casts three shadows, each one pointing in a different direction. Streetlights, porch lights, illuminated signs, and the headlights of passing cars all create a wonderful array of constantly changing shadows. As you walk past a light source, your shadow will change position--falling behind you as you approach the light, beside you as you pass it, and before you as you leave it behind.

We can't tell you exactly why watching shadows is so interesting. Perhaps it has something to do with the malleability of shadows, shape-shifting doppelgangers that dog our every step. There's something fascinating about watching your shape shift and change. But we suggest you go for a walk in the city at night and watch for shadows.

Reflections on Transparency

One evening, while riding the N Judah, a municipal train that rattles and sighs through the streets of San Francisco, Pat became aware of reflections in a new way. The train's overhead lights were bright and the train's windows reflected the train's interior—the bright orange walls, the advertisements inside the train, and the weary faces of her fellow commuters.

Through the reflections, Pat could catch glimpses of the outside world--but that outside world collided with the world she could see reflected in the windows. People strolling past a brightly lit doorway walked without hesitation through the reflection of Pat's head. When the train stopped, a woman in a white rain hat stepped outside and disappeared, becoming invisible in the gloom. But her white hat didn't disappear. It bobbed cheerfully away, moving through the reflected newspaper of the man behind Pat.

When light shines on window glass, about 92 percent of the light passes through the glass and about 8 percent reflects back. At night, looking at the window of a brightly lit room, you can see your own reflection--the 8% bouncing back is brighter than the 92% coming in from the gloom outside. A stranger outside your house can see you--the 92% coming through from the room's interior is brighter than the 8% of the light outside bouncing back to him. At some balance point, when the lighting on both sides of the glass is just right, reflected light and transmitted light compete.

Ever since that moment on the train, Pat has kept watch for those balance points where reflections compete with reality. These moments amuse her. The world is a changeable place, she says, and clarity sometimes depends on your point of view.

Floaters in your eyes

During the long hot Connecticut summers when she was in elementary school, Pat spent many hours lying in the grass of her family's suburban lawn and watching the things that drifted in the cloudless blue sky. The thing she watched most frequently looked like a tangle of transparent threads that had come together in a shape that looked vaguely like a stick figure of a horse. Early in her sky watching, she realized the drifting shape was not in the sky, but in her own eyes.

It wasn't until Pat came to work at the Exploratorium that she found out that drifting horse shape was a bit of junk floating in the liquid in front of her retina. These bits of junk, generally known as =floaters,= are often the remnants of blood cells or structures that were part of the eye when it was developing.

You can easily check your own eyes for floaters. On a clear day, stare up at the blue sky and relax. (Pat suggests lying down on a grassy hillside and imagining you are on summer vacation, but that part isn't really necessary.) Chances are good that the blue will be marred by at least one floating translucent form. Once you've spotted a floater, you can figure out which eye it's in by closing one eye, and then the other. If the floater disappears when you close an eye, that's the eye it's in.

Seeing floaters requires you to pay attention to visual stimuli that you usually ignore. To learn about other things that you can see in your own eyes, check out Pat's web site at In the meantime, use this as an excuse to do a little sky watching—and maybe take a well-earned nap.



While Pat contemplates soothing sights like dappled shade and reflections, Paul urges you to watch lightning, one of nature's great shows. He remembers leading a group of hikers up Cotopaxi, a glacier-covered mountain in Ecuador. Cotopaxi is best climbed at night; by day, the hot equatorial sun turns the snow to slush, making climbing difficult. Paul says, "I noticed flashes of light off to the east. Huge cumulonimbus clouds carpeted the Amazon basin. They were alive with lightning flashes. The best part was that I was above them. I had seen images of Amazonian lightning sent to earth by space shuttle astronauts, here I was getting the same view and I had earned it by walking up a hill. Presented with this spectacle, I just stopped and watched. To this day I'm glad I took the time to look. I still get chills down my spine when I think about what I saw that night."

You don't need to go to Ecuador to find lightning. In most areas of the United States, the lightning will come to you. The safest place to watch lightning is from inside a metal car, which acts as a Faraday cage, protecting you while conducting the electrical charge to the ground. And being safe is a good idea while watching lightning: The moving charges in a lightning bolt heat the air to ten times the temperature of the surface of the sun, rip air molecules apart into ions, and create a hot incandescent plasma that snakes its way to the ground.

While you are contemplating the power of the lightning bolt, notice the path of lightning. Is the bolt forked, with branches that stop before reaching the ground? If it is, it's the first bolt. Some flashes repeat more than a dozen times in a second. Subsequent bolts follow the path that the first bolt has blazed to the ground, ignoring the dead end branches.

Notice the color of the flash. Paul reports seeing amazing blue flashes during snowstorms, colored by ice crystals in the air. (For a real psychedelic light show you can even look at a lightning storm through a diffraction grating. Inexpensive ones can be purchased from Edmund's Scientific). To scientists, the spectrum of the lightning reveals the identities of ions excited by the bolt and their temperatures.


While you're watching lightning, we suggest you pay attention to the thunder as well. The thunder from the nearest part of the bolt arrives first followed by sounds from more and more distant parts of the bolt. The thunder paints an acoustic image of the shape of the bolt. When a bolt jogs to the side so that it is perpendicular to your line of sight, it makes an extra loud boom since all of the sound from that part of the bolt arrives together.

In September of 2000, when he was taking a break from writing this column, Paul camped in the Cirque of the Towers, a high glacial basin in Wyoming. A lightning storm blew in quickly and he retreated to the tent he had pitched in a moderately lightning-safe place. To follow the approach of the storm, he counted the seconds between the lighting and the thunder. The distance is 1 kilometer for every 3 seconds of delay. Many flashes were followed by thunder less than a second later, indicating that the flash was less than a football stadium length away.

Listening to the thunder, Paul noticed something he had never heard before. There would be a flash of lightning, followed a second later by a relatively quiet boom of thunder. Then, after 2 more seconds, the thunder suddenly became loud. Flash . . . booomBOOOOM. This was a puzzle: Why would the closer lightning channel produce less sound? Then Paul realized that the lightning was striking behind a ridge. The sound waves from the nearest sounds were bending or diffracting over the ridge, greatly reducing their sound intensity. After listening to thunder for more than five decades Paul had heard a new effect. It always pays, he says, to look at and listen to nature.


Paul seems to do a lot of climbing at night. While hiking up Mt. Hood at night, he noticed strange flashing lights on the snow in front of him. He looked up into the clear sky to see meteors! It was a night of the Perseid Meteor shower, and the snow was reflecting the meteor light. A couple of times the meteors ended their flight in a bright explosion known as a bolide. Perhaps the most startling thing is that the explosions are completely silent. You can wait and wait and the sound will never get to you. (Remember: in space, no one can hear you scream.)

The best to time to watch meteors is after midnight on a night when there is no moonlight. Then you are on the leading side of the earth as it sweeps through space, pulling meteors into its gravitational field. The bright flashes of light that we see are caused by bits of dust and debris, burning up as they drop through Earth's atmosphere. Most of the objects that make meteors are bits of debris blasted off of asteroids during collisions or shed by comets. They orbit the sun, traveling at tens of kilometers per second and flash into incandescence when they hit the earth's atmosphere.

When you spot a meteor, notice its path. During a meteor shower, the meteors will all seem to radiate from one point in the sky. Meteor showers are named after the constellation containing that point. The Leonids seem to come from Leo, for example, and Geminids seem to come from Gemini.

Some meteors move faster than others across the sky. Paul has seen a wonderful bright green light from a meteor, possibly an indication that the meteor was passing through a layer of oxygen, which glows green when it absorbs and re-emits energy. Sometimes meteors leave "smoke" trails behind them. These trails get blown around into fantastic shapes by high altitude winds.

Patience is required to watch for meteors. You may have to watch for half-an-hour to see just one. Relax and enjoy the other sights and sounds of the night sky. Rejoice when you see an extraterrestrial visitor.

For instant gratification, take a magnet outside and drag it around near the outflow of your downspouts. It will collect bits of iron and iron oxide. Many of these bits came from meteors.


Whatever it takes, we suggest you go and see a total solar eclipse. Paul has seen four, Pat has seen one, and they have all been spectacular and different. Paul once traveled to Chile to see an eclipse. Watching the eclipse over an erupting volcano next to a herd of Alpacas made it clear he wasn't in Kansas anymore!

When the shadow of the moon falls on you and you look up to see the sun eclipsed in the sky, your body responds physically. One of the things your body knows is that the sun is not supposed to vanish from the clear sky in the middle of the day.

Just before an eclipse, the sunlight turns a very strange yellow orange. Animals begin to settle down into twilight behavior. People run around setting up telescopes and getting cameras ready. Paul lets others photograph and record the eclipse; he just settles back to watch. He uses a safe viewing filter during the partial eclipse, but as totality starts, he looks with his naked eyes. It's awe inspiring to see a white spider of the solar corona spreading out from the sun into a black sky.

When you are watching an eclipse, glance at the ground just a minute before totality, when the sun has been reduced to a narrow crescent. During a recent eclipse in Turkey, Paul was amazed to see dark shadows creeping across the ground. These are shadow bands, caused by ripples in the air. Just as ripples on the surface of a swimming pool cover the bottom of the pool with patterns of light and dark, so ripples in the density of the air, caused by difference in temperature, can make bands of shadows on the ground. These bands are only visible when the sun is reduced to a smaller size than normal, as it is during a solar eclipse.

Last word

We hope that you will continue exploring the world around you. Take the time to stop and look and marvel at the strange and marvelous ways the universe works. Have fun!


The latest book by Pat Murphy and Paul Doherty is Traces of Time, published by Chronicle Books. You can read more of Pat's work at and more about Paul's adventures at

===THE END===

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