Shadow Lengths Through the Day
A class records the length of a flagpole's shadow every hour on a sunny day. At 8 a.m. the shadow is 12 m long. At noon it is 4 m. At 4 p.m. it is 11 m. The students also record that the shadow points west in the morning and east in the afternoon. A student claims the Sun moves around Earth, which is why the shadow changes. Another student says the shadow changes because Earth rotates. The class must decide which explanation fits the evidence.
1. Explain the pattern of shadow length and direction using Earth's rotation and the Sun's apparent motion.
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Earth rotates on its axis once about every 24 hours. As it turns, the Sun appears to move across the sky from east to west. In the morning the Sun is low in the east, so the flagpole's shadow points west and is long. Near noon the Sun is high, so the shadow is short. In the afternoon the Sun is low in the west, so the shadow points east and is long again. The changing length and direction both fit this explanation.
2. The student says the Sun moves around Earth. How could the class use the shadow data to respond, and what other evidence would help?
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The shadow data alone show a repeating daily pattern, but they do not by themselves prove which object moves. The class should combine the pattern with other evidence. For example, stars appear to circle the sky each night, and spacecraft and satellite observations show Earth rotating. The simplest explanation that fits all the evidence is that Earth rotates while the Sun stays nearly fixed relative to the solar system. The class can say the shadow pattern is consistent with Earth's rotation and that the rotation explanation fits a wider set of observations.