Astronomy Basics
AU, Lunar Distance, and Light-Years Explained
Choose the right scale for a Moon crossing, an asteroid approach, a planetary orbit, or a distant star.
Prepared with AI assistance using the primary sources linked below. How Astroboat works
Use nearby yardsticks in the solar system
One astronomical unit, abbreviated AU, is exactly 149,597,870.7 kilometres. It is based on the scale of Earth's average distance from the Sun. Saying an object is several AU away is usually easier to follow than a long string of kilometres.
Lunar distance is another useful scale: roughly the average Earth-Moon separation, about 384,400 kilometres. It makes an asteroid approach easier to picture. Neither Earth's distance from the Sun nor the Moon's distance from Earth stays perfectly constant.
A light-year is a distance
A light-year is the distance light travels in one year, about 9.46 trillion kilometres. Despite its name, it is not a unit of age or speed. Light-minutes and light-hours use the same idea for shorter distances.
If light from a nearby star takes ten years to reach us, we see that star as it was ten years ago. For very distant galaxies, cosmic expansion makes the relationship between light travel time and present-day distance more complicated than this nearby example.
Keep the definition attached to the number
An asteroid's closest distance to Earth is not its distance from the Sun. A spacecraft's distance from Earth is not the length of its whole journey. Before comparing two figures, check the reference body, time, and units.
In Astroboat's Asteroid Watch, use the displayed unit labels and source record together. As a simple exercise, compare an approach given in lunar distances with the same approach in kilometres. The conversion builds intuition without turning an astronomical use of close into an impact prediction.