Astronomers typically use a number of different length units for different objects. The length unit used is typically determined by two criteria. The distances are closely related to the cosmic distance ladder.
- the first is that the unit create manageable numbers
- the second is that the unit be easily derivable from observation
The following table lists the typical units used for various astronomical distances:
| Astronomical Range
| Typical Units
|
| Distances to satellites
| kilometres
|
| Planetary distances
| astronomical units
|
| Distances to nearby stars
| light years, or parsecs
|
| Distances at the galactic scale
| kiloparsecs (however light years is more common in general use)
|
| Distances to nearby galaxies
| megaparsecs (however light years is more common in general use)
|
The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift. The reasons for this are that converting redshift to distance requires knowledge of the Hubble Constant which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of space-time allows one to come up with multiple definitions for distance. For example, the distance as defined by the amount of time it takes for a light beam to travel to you is different from the distance as defined by the apparent size of an object.