Junior Interns at the Planning Office
1Every summer, the planning office in the city of Brannock, at 40° north latitude, hires two eighth graders as junior interns. This year Dashiell Mbeki joined the team that writes the city's water and energy plan. Much of the city's daytime electricity comes from a large solar farm on the east side. The pumps that move water from the river to the treatment plant use a lot of that electricity.
2Next spring, a solar eclipse will cross the region on a weekday afternoon. The narrow path where the eclipse is total passes about 300 km south of Brannock. In Brannock itself, the Moon will cover about 80% of the Sun's disk at the peak. The office's planner, Ms. Ilse Varga-Bello, showed Dashiell a forecast of the solar farm's output for a clear eclipse day (Table 1). On a clear afternoon without an eclipse, the farm makes about 100 megawatts (MW) at each of those times.
3To explain the forecast at a council meeting, Dashiell built a model in a dark room. A lamp stood for the Sun, a globe for Earth and a table tennis ball on a stick for the Moon. When he held the ball between the lamp and the globe, the ball cast a small, very dark spot on the globe, surrounded by a much wider, paler shadow. When he moved the ball to the far side of the globe, the ball slipped into the globe's shadow.
4For the yearly plan, Dashiell also compared the solar farm's monthly energy with the water the city pumps (Part 2). He learned that Earth is closest to the Sun in early January, about 147 million km away, and farthest in early July, about 152 million km away. Then he taped a tiny solar cell to the globe at Brannock's latitude, kept the globe's axis tilted toward the same wall of the room, and carried the globe around the lamp. At each stop he turned the globe so the cell faced the lamp, as it does at noon. With the north end of the axis leaning toward the lamp, the cell's meter read 41 mA. With it leaning away, the meter read 18 mA. At the two positions where the axis leaned to the side, it read 30 mA.