The Observatory Above Town
1Above a small mountain town, a solar observatory sits on a rocky ridge where the air is thin and dry. The town below sits in a steep valley where road crews watch for rockslides after storms, but up on the ridge the sky is often clear, and there is less air above the telescope to blur the view. On summer Saturdays it opens to visitors, and eighth grader Oskar Ferreira-Lund helps as a volunteer. The observatory's telescope never sends sunlight into anyone's eye. Instead, it projects an image of the Sun onto a white screen in a dark room, which is the only safe way the guides let visitors look at the Sun. Looking at the Sun directly, or through binoculars or a telescope, can badly damage the eyes.
2The guide, Ms. Marguerite Tshabalala, adjusts the telescope so that the Sun's image on the screen is exactly 139.2 mm wide. Because Earth keeps turning, the image would slowly drift off the screen, so a motor turns the telescope to follow the Sun. The real Sun is about 1,392,000 km wide, so every 1 mm on the screen stands for 10,000 km on the Sun. One morning Oskar traced the dark sunspots on paper taped to the screen and measured how wide each one was (Table 1). Sunspots are cooler, darker patches on the Sun's surface. Most last from a few days to a few weeks.
3Visitors often ask how the sunspots compare with Earth, so Ms. Tshabalala keeps a second table by the screen (Table 2). It lists the diameters and masses of a few objects, with each mass compared with Earth's mass. She also reminds visitors that the Moon orbits about 384,000 km from Earth.
4Oskar wants to build a lobby display at the same scale as the screen image. He plans to show Earth as a tiny dot and to measure, out the door and down the hall, how far from the Sun image the dot would have to sit. Earth is about 150 million km from the Sun. He also wants a sign that compares the Sun with the planets by mass, using the numbers in Table 2.