A star is defined as being one parsec away if it shows what specific parallax shift over six months?

Answer

A shift of exactly one arcsecond

A parsec is defined as the distance to a star that exhibits an apparent positional shift (parallax) of exactly one arcsecond when viewed from opposite sides of Earth's orbit over six months.

A star is defined as being one parsec away if it shows what specific parallax shift over six months?

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