(Incidentally that's what defines a "parsec" -- a unit of distance of about 3 light years. If you see a parallax of one arcsecond ("par-sec"), from opposite sid

(Incidentally that's what defines a "parsec" -- a unit of distance of about 3 light years. If you see a parallax of one arcsecond ("par-sec"), from opposite sides of the earth's orbit around the sun (2 AU apart), that something is 1 parsec away. If it's 10 parsecs away, the parallax will be 0.1 arcsec -- the more distant the object, the less it appears to shift. This works to about 0.001 arcsec of precision (distance limit of ~1,000 parsecs), and soon down to 20 microarcseconds (~50,000 parsecs). https://en.wikipedia.org/wiki/Parsec https://en.wikipedia.org/wiki/Parsec https://en.wikipedia.org/wiki/Hipparcos https://en.wikipedia.org/wiki/Hipparcos https://en.wikipedia.org/wiki/Gaia_%28spacecraft%29 https://en.wikipedia.org/wiki/Gaia_%28spacecraft%29 This is the lowest rung of the "cosmic distance ladder": https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://terrytao.wordpress.com/2010/10/10/the-cosmic-distance-ladder-ver-4-1/ https://terrytao.wordpress.com/2010/10/10/the-cosmic-distanc...