Astronomical units of length

Astronomers typical use a number of different length units for different objects. The length units used is typically determined by two criteria. This distances are closely related to the cosmic distance ladder.

• the first is that the unit create managable numbers

• the second is that the unit be easily derivable from observation

distances to satellites are typically measured in kilometers

planetary distances are typically measured in astronomical units

distances to nearby stars are measured in light years or more commonly parsecs

distances at the galactic scale are typically measured in kiloparsecs

distances to nearly galaxies are typically measured in megaparsecs

distances to distant galaxies are typically not quoted in distances at all but rather in terms of redshift. The reason for this is 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.