Seeing the Light

One-liners like “moving clocks run slow” and “moving objects appear shorter” are often used in the teaching of special relativity. However, they may lead to alternative conceptions by blurring the distinction between visual observation and coordinate measurement of events. In this paper, we introduce a graphical reasoning tool to show how a clock can be seen to run slower or faster depending on whether the observer is approaching or receding at relativistic speeds. The example is meant to encourage and support teachers to discuss with their students the distinction between seeing and measuring.