In this investigation, students use "point-source" light, light meters, and graphing software to quantify the reduction in light over distance. Through careful measurement of light received at several distances, students discover the best fit of the... (View More) data is the inverse square rule. Using this rule, students then calculate the distance between the light source and the light meter at random placements. Finally, students extend this principle to model the manner in which distances to Cepheid variable stars are measured. The distance between the Cepheid (here the light source) and the Earth (the light meter) can be determined by comparing the output of the source to the amount of light received. An historic scientific breakthrough occurred when the period-luminosity relationship of Cepheids was quantified throughout the early 1900s. This breakthrough allowed astronomers to gain a more correct understanding of the dimensions of our galaxy and the universe beyond. This activity is part of the "Cosmic Times" teacher's guide and is intended to be used in conjunction with the 1929 Cosmic Times Poster. (View Less)