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.
| Distance | |||
| Light-travel Time | (AU) | (kilometers) | (miles) |
| 1 light-second | 0.0020 | 299,792.458 | 186,322.224 |
| 1 light-minute | 0.1202 | 17.98 million | 11.18 million |
| 1 light-hour | 7.2143 | 1.08 billion | 670.76 million |
| 1 light-day | 173.14 | 25.9 billion | 16.098 billion |
| 1 light-week | 1212.0 | 181.3 billion | 112.69 billion |
| 1 light-month (30 days) | 5194.3 | 777.06 billion | 482.95 billion |
| 1 light-year | 63240.2 | 9.46 trillion | 5.89 trillion |
| Object | Light-travel distance |
| Mercury | 3.22 light-minutes |
| Venus | 6.01 light-minutes |
| Earth | 8.32 light-minutes |
| Mars | 12.7 light-minutes |
| Jupiter | 43.3 light-minutes |
| Saturn | 1.32 light-hours |
| Uranus | 2.66 light-hours |
| Neptune | 4.16 light-hours |
| Farthest comets | 1.58 light-years |
| Proxima Centauri (nearest star) | 4.3 light-years |
| Orion Nebula | 1,500 light-years |
| Center of the Milky Way | 26,000 light-years |
| Andromeda Galaxy (farthest naked-eye object) | 2.36 million light-years |
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?