Which distance measurement is the longest?

Answer 1

Depends on the field of interest. Certainly the greatest lengths/distances are found in astronomy and the longest BASIC distance is the "parsec," equal to about 3.26 light-years. However, metric prefixes allow for longer DERIVED units, such as Kiloparsec (useful if you're describing the arms of the Milky Way galaxy) and Megaparsec (for distances to other galaxies). By extension, the current maximum prefix is "Yotta," so one could theoretically speak of "Yottaparsecs," though such would extend far beyond the known universe and consequently wouldn't be of much use.

Of measures on a more human scale, the longest might have been the Swedish Mile, equal to about ten statute miles or sixteen kilometers. Of course, such units were highly local in their application and indeed, one might develope an "idiosyncratic mile" of any length.

Within the Metric System, or SI, the longest measure originally adopted was (and in theory, still is) the myriadometer, or ten kilometers.

Answer 2

A parsec, which is about 3.26 light years.

Answer 3

Kilometres is the longer distance.

1 km = 1000 m

Answer 4

roughly 13.5 billion light-years