Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. This is because the Earth's atmosphere limits the sharpness of a star's image. The parallax method is the fundamental calibration step for distance determination in astrophysics however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01″, and thus to stars no more than 100 pc distant. To the nearest meter, the small-angle parsec corresponds to 30 856 775 814 913 673 m. As the astronomical unit was defined by the IAU (2012) as an exact SI length in metres, so now the parsec corresponds to an exact SI length in metres.
This differs from the classic inverse- tangent definition by about 200 km, i.e. The length of the parsec used in IAU 2015 Resolution B2 (exactly 648 000 / π astronomical units) corresponds exactly to that derived using the small-angle calculation. if Distance earth-sun = 1 au, unit for Distance star is in astronomical units if Distance earth-sun = 1.5813 ×10 −5 ly, unit for Distance star is in light-years). The calculated stellar distance will be in the same measurement unit as used in Distance earth-sun (e.g. Where θ is the measured angle in arcseconds, Distance earth-sun is a constant ( 1 au or 1.5813 ×10 −5 ly). This corresponds to 648 000 / π astronomical units, i.e. The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 au subtends an angle of one arcsecond ( 1 / 3600 of a degree). 30.9 trillion kilometres (19.2 trillion miles). The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (au), i.e. A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale)
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