One evening recently I was sitting in my living room with the TV on and the lights off. I was vacantly watching a shadow on the ceiling when it struck me that the light from the TV had enough power to create moving shadows that changed as images moved on the screen. It dawned on me that the TV was a light source, actually a bunch of lights, called pixels (for "picture elements").
Hmmm. How could this be demonstrated with my computer? I did some quick-and-dirty programming to move a white sprite across a dark screen, and sure enough, a pencil propped against the screen created a moving shadow.I then remembered how my kids had spotted a sundial recently, and asked how it worked. Here was a chance to kill two birds with one stone.
"Sundial 128," is the "lab report." Scrounge around your house and find a piece of cardboard about a foot square, a nail at least three inches long and a pencil. Jab the nail through the cardboard near the center. This nail is the gnomon (the part that casts the shadow) on your sundial. The cardboard is the dial's face. Place the cardboard just under the front of your monitor with the nail centered with the screen. Run the program and darken the room.
You have three options: Set, Guess and Else. Press S to set your timepiece, and you'll see the morning sun (a white ball) rise in the east (the left side of your screen). The time of day will be displayed along with it: 5:30, 5:35, and so on up to 6 a.m. Then everything will freeze to let you make your first marking.
Look down at your sundial. You'll see that the standing pointer does indeed cast a shadow, not sharp, but visible. Trace the shadow-line with a pencil. Hit any key to move the sun along by hour, tracing the shadow as it moves until you have a complete, calibrated sundial (6 a.m. to 7 p.m.). Whenever you run the program, you'll need to recalibrate by moving the cardboard until you get the lines to match up with the times.
When you select Guess, the sun will again make its ascent, only it will stop along the way and you will be asked to guess the time using the cursor keys, which step through hours and minutes in five-minute increments. Make your selection and press Return to see the results. The third option is just for the heck of it, and lets you watch the sun rise and fall with the time displayed along with it.Doing this little experiment prompted me to think of other projects that use the monitor as a light source: an animated shadow show or a game that sends light beams to a map to give clues.
Have you done a nifty home experiment with your Commodore 128? Send me a description of it, including your name, address and telephone number to the RUN editors using The Letters To the Editor option on this ReRUN disk. If they get enough responses I'll devote an upcoming 128 Mode column to them.
