How this calculator works
Annual parallax is the apparent displacement amplitude associated with an Earth-orbit baseline of one astronomical unit, not the full six-month displacement. The IAU parsec convention gives distance in parsecs as the reciprocal of the parallax in arcseconds. This is the small-angle astrometric relation, not a tangent-based nearby surveying formula. Proxima uses Gaia DR3; the other presets reproduce historical van Leeuwen 2007 Hipparcos parallaxes, without uncertainty propagation. These are illustrative catalogue fits, not current best distances. Binary orbital fits can bias results, particularly the separate Alpha Centauri components, and bright supergiant parallaxes are uncertain. Use a custom current measurement for research. Negative measured parallaxes require statistical inference and cannot be inverted as physical distances.
Formula
d(pc) = 1/p(arcsec); d(m) = au/p(rad); 1 pc = 648000 au/pi.
Worked example
A parallax of 0.1 arcsec gives 10 pc, about 32.616 light years. A parallax of 1 arcsec defines 1 pc.
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Frequently asked questions
Should I enter the total shift over six months?
No. Use annual parallax, the angular semi-amplitude. A full peak-to-peak shift is twice as large.
Why reject zero or negative parallax?
Direct inversion requires a positive measurement. Noisy or negative catalogue values need a distance inference with uncertainty.
Is a milliarcsecond an arcsecond?
A milliarcsecond is one thousandth of an arcsecond. Enter 0.001 arcsec for 1 mas.