I know many people who refuse to wake up or get out of bed in the morning until the gentle rays of the Sun warm their face. What is not apparent to the average person is that those rays of light travel at the finite speed of 186,000 miles per second. That those rays of light transverse about 93 million miles from the Sun to the Earth. It has taken those same rays of light 8.3 minutes ((93000000/(186000 mps x 60) = 1116000 mpm) =8.3). The time that it takes light to travel from the source (the Sun) to an observer is called “lookback” time. In this case the lookback time is 8.3 minutes.

When looking at distant stars we are seeing the light of those stars that has finally arrived here on Earth that in some cases have taken over 2 billion years to transverse the void of space. We are seeing these stars as they were 2 billion years ago.

1160000

The time it takes light to travel from a source to an observer. Light travels at the finite speed of 300,000 kilometers per second. The Sun is so distant that light takes 8.3 minutes to travel from it to Earth. Hence, the Sun has a lookback time of 8.3 minutes; we see it as it was 8.3 minutes ago. The Andromeda Galaxy has a lookback time of 2 million years; we see it as it was 2 million years ago

Some Great Links:
http://abyss.uoregon.edu/~js/glossary/lookback_time.html
http://www.site.uottawa.ca:4321/astronomy/index.html#lookbacktime
http://www.windows.ucar.edu/tour/link=/the_universe/Lookback.html