Continued 2 In the northern hemisphere, a sundial must be aligned with true north. In the southern hemisphere, it must m be aligned with true south. 3 The number sca al e on the sundial and the shadow w cast by the rod are used to tell time. The shadow on the s su ndial in the photograph is abou ut 1/4 of the way between XII (twelve) and I (one). So, the time is about 12:15, or fifteen minutes n past noon. 4 Notice the number b scale begins at VI (six) and ends n at IV (four). Why do you think that is? The sundial only y works when the Sun is shining, so at six in the morning and four at night for this location there is not enough sunlight to cast a shadow. For the same reason, the number scale do oe s not include nighttime numbe er s. Try It! A sundial is a simple m device. It can be created from m three basic items: a flat surface, a thin object to cast a shadow, o and a number scale. Collect a a flat piece of cardboard and a pencil, straightened paper clip, c or similar thin object. Devise a plan for creating a sundial using these materials. Then n, carry out the plan to create a basic sundial. How will you construct the sundial? How will you o create the timescale on the sun nd ial? Review and Reflect Once the plan is carried out, reflect on it. Think about it. Evaluate how well it worked. Did the algorithm solve the problem? Was it a success? If not, revise it, and try again. Consider Petrus Apianus’s plan to tell time by the location of the Sun. Was the plan successful? Yes. A sundial can be used to tell the approximate time. Think about any problems with the plan. Are there any times when a sundial cannot be used to tell time? It does not work at night or on cloudy days. Consider Frederick Smith’s plan for overnight delivery. Was the plan successful? Yes. FedEx set the standards of speed and reliability for delivery of letters and packages. Section 2.2 Solving Problems A computer does only what a coder tells it to do. It is not capable of original thought. Humans must write the algorithms that a computer will use. By the time a computer program is written, the way to solve the problem is known. Humans understand the problem. Humans devise the plan and write the algorithm. Computers use the algorithm to produce the answer. Following are two examples of problems that can be solved using an algorithm. The first example is calculating a perfect square. The second example describes how to play the Guess My Number game. You can improve your problem-solving skills through practice in solving many different types of problems. Copyright Goodheart-Willcox Co., Inc. Chapter 2 Solving Problems 27