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:
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.
Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).
Units of length may be based on lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed object.
Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomicalunit, the light-year, and the parsec.
Astronomers use a number of different lengthunits for different objects.
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.
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