Light-Travel Time and Distance

The Union society  measures distance in light-travel time rather than in kilometers or miles. Light-travel time refers to the distance light travels over a given time. We know light travels at 299,792 km/sec (186,322 miles/sec). At that speed, the light-year becomes a convenient unit of distance in the Milky Way. In this appendix, we list common conversions between light-travel times and distances that we're familiar with.

The table above lists some light-travel-time distances to a few well-known places. These are measured from the Sun. Included are the major Solar System objects, including Earth, which is about 8 light-minutes from the Sun. Think about this for a minute. If the Sun's light takes 8 light-minutes to reach Earth, then something catastrophic could happen on the Sun and we would not know for 8 minutes. The nearest star to the Sun is more than 4 light-years away. Look to that star in the sky tonight and you're seeing it as it was 4 years ago. The center of our Galaxy is 26,000 light-years away. The light we see from there left when our ancestors were painting primitive scenes in caves. The farthest object we can see with our eyes, in a dark sky without the aid of a telescope, is the Andromeda Galaxy, more than 2 million light-years away. What was happening on Earth 2 million years ago?