Q:

Why is that for 1 parsec such an odd value has been chosen(i.e. 3.26 light years)? For other measurement units we generally have values such as 10,100 or something like that.

A:

The use of a parsec as the unit of length stems from the way in which astronomers measure the distance to nearby stars. One parsec is the distance at which the parallax of a star is equal to one arc second.

As the Earth orbits the Sun, the nearer stars appear to shift against the background of distant stars. To see how this works, hold your finger up at arm's length in front of an object about ten feet away's length The parallax of a star is apparent shift of its position due to the orbit of the Earth around the Sun.

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