Stellar Parallax Distance Calculator

How it is calculated

The tool uses d = 1 / p, where p is the annual parallax angle in arcseconds and d is the distance in parsecs. A parsec is by definition the distance at which the parallax is exactly one arcsecond. The distance in light-years is then d × 3.261563777. Both results are rounded to two decimals.

The parallax must be greater than zero; 0 or a negative value gives no result, since a zero shift would mean an infinite distance. Enter the angle in arcseconds, not milliarcseconds or degrees. The formula holds well for small angles, which is the case for every star, and the tool does not account for measurement error.

Common mistakes

• Entering the parallax in milliarcseconds: catalogues often list values like 100 mas, which must be typed as 0.1 arcseconds, otherwise the distance is 1000 times too small.
• Using the angle of the star's full back-and-forth shift; the parallax is the shift measured against Earth's orbital radius (half of the six-month displacement).
• Mixing up parsecs and light-years: 1 pc is about 3.26 ly, so the two lines of the result are not interchangeable.
• Trusting the distance from a tiny parallax; when the angle is close to its measurement error, 1 / p can be very uncertain.
• Entering a negative or zero parallax from a noisy catalogue entry; the tool returns no result for it.