5 Lookback Time (Assignment)

Space is vast, and there are huge, empty, stretches of it between us and everything that we observe in the sky. To cope with these distances, astronomers invented different units of distance.  For objects in our Solar System, we use astronomical units.  The average distance from the Sun to the Earth is one astronomical unit (au).  For stars and galaxies, astronomers use either light years or parsecs. A light year is the distance that light travels in one year; note that light year is a measure of distance, not of time.

Watch the video to see the distance to some relatively “nearby” astronomical objects as measured by light travel time.

An important concept that is related to light travel is lookback time.  Unlike the nearby stars, the light from a distant galaxy or quasar left its source a very long time ago.  The time that light has taken to travel from its source to us is called the lookback time.  By collecting light from distant objects, we can look backward in time to when the Universe was younger, which allows us to study the evolution of the Universe and the objects (stars, galaxies, black holes, etc.) that form in it.