This simulator shows how the finite speed of light and the great distances to astronomical objects cause us to observe things as they were in the past.

The top panel shows an observer who is looking at a distant star. The distance betwee the star and the observer is given in light-years (ly), which is the distance light travels in one year.

The thought bubble above the observer's head shows what they see at the time indicated by the cursor in the timeline. When the go supernova button is pressed the star will explode and the simulation will start running. An expanding circle represents the light from the explosion. Only when the circle reaches the observer do they know that the star has exploded, and afterward, they no longer see the star.

This simulator is an HTML5/Javascript re-implementation of the Lookback Time simulator that was originally developed as part of the Nebraska Astronomy Applet Project.

The original simulation, along with related teaching materials, may be found at: https://astro.unl.edu

This simulator is part of the Foothill AstroSims project, which is aiming to re-implement, in HTML5, Flash-based simulations that are used in Foothill College Astronomy courses.

Similar projects include work done by Nik Nyby at Columbia University (https://github.com/ccnmtl/astro-simulations) and Kevin Ditts at the University of New Mexico (https://physics.unm.edu/Courses/Rand/applets/).

Funding for Foothill AstroSims provided by an Equity & Innovation Grant from Foothill College.