Looking Back in TimeOverview: Calculate time for sunlight to reach Earth and consider other time-based distance measurementsLearning Goal: Understand that light takes time to travel Video Link: New View of the Universe How do astronomers discover the history of the universe? What we detect in the night sky is essentially a snapshot of the past. To illustrate this concept, ask students when they think they would see the Sun "go out" if it were extinguished at exactly 12 noon. Remind them that the Sun is 150 million km away and that light travels about 300,000 km per second. (Earthlings would see the Sun vanish at about 12:08 P.M.) To explore why we see a delayed image, brainstorm other examples or analogies of delay in transmission time. For example, after you shut off the faucet, water still pours out of a garden hose for a few seconds. Ask students how far light would travel in a year if it takes eight minutes to go 150 million km. That distance, 9,450,000,000,000 km, is a light-year. Why would people invent this measure of distance? Brainstorm examples from students' experience of using time to measure distance (e.g., a 15-minute walk to school). Invite students to invent other speed-time units to measure distance -- a slug-minute, for example. What problems could this method have? (A particularly energetic slug would throw you off.) Discuss how the constancy of light's speed makes light-years extremely accurate. This exploration of light and distance can help students grasp the magnitude of Edwin Hubble's discoveries, seen in the "New View of the Universe" video segment. Physics and Astronomy Program Contents Atomic Ethics Stranger than Fiction? In-Depth Investigation: Universal Proportions Home | Resources for Educators Menu | Educator's Guide Contents | Help WGBH | PBS Online | Search | Feedback | Shop © 1998 WGBH