What is annual parallax?

Annual parallax is the maximum angular displacement of a star from its mean position as observed over one year, caused by Earth's orbit around the Sun. It equals half the total angular shift seen over six months. A parallax of 1 arcsecond defines a distance of exactly 1 parsec.

A parsec (pc) is the distance at which a star would show an annual parallax of exactly 1 arcsecond. It equals approximately 3.2616 light-years or 206,265 astronomical units. The name comes from "parallax of one arcsecond."

Why does parallax only work for nearby stars?

Beyond roughly 10,000 light-years (about 3,000 pc), the parallax angle shrinks below a few hundred microarcseconds — the practical detection limit even for the Gaia space telescope. For more distant stars, astronomers use standard candles such as Cepheid variables or Type Ia supernovae.

What parallax does Proxima Centauri have?

Proxima Centauri has the largest known stellar parallax at 768.5 milliarcseconds (0.7685 arcseconds), placing it at about 1.30 parsecs or 4.24 light-years — the closest star to the Sun.

How do I use Gaia data with this tool?

Gaia publishes parallaxes in milliarcseconds (mas). Simply select "mas" as the input unit, paste the parallax value from the Gaia archive, and the tool converts it to parsecs and light-years automatically.

What does a negative parallax mean?

In real catalogs such as Gaia, a few stars show a slightly negative parallax due to measurement noise when the true parallax is very small. A negative value has no physical meaning for distance and this tool will flag it as invalid.

Star Distance via Parallax

Enter a star's annual parallax in arcseconds and instantly get its distance in parsecs, light-years, and astronomical units.

Parallax Input

Parallax Unit

arcsec = arcseconds | mas = milliarcseconds | µas = microarcseconds

Annual Parallax

arcsec

Parallax Uncertainty (optional)

arcsec

Used to compute the distance uncertainty range.

Famous Stars (arcsec)

Enter a parallax angle and click Calculate.

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Summary

Stellar parallax is the apparent shift in a star's position against the background sky as Earth orbits the Sun. By measuring the half-angle of that shift — called the annual parallax (p) — astronomers can calculate distance directly from the formula d = 1/p, where d is in parsecs and p is in arcseconds. This tool converts any parallax measurement into parsecs, light-years, and astronomical units so you can immediately grasp how far away a star lies.

How it works

Enter the star's annual parallax angle in arcseconds (milliarcseconds and microarcseconds are also accepted).
Select the input unit: arcseconds, milliarcseconds (mas), or microarcseconds (µas).
Click Calculate to apply d = 1/p and convert the result into parsecs, light-years, and AU.
The result panel shows the distance in all three units along with an uncertainty estimate if you supply a parallax error.
Use the preset famous-star buttons to explore well-known examples instantly.

Use cases

Convert Hipparcos or Gaia catalog parallaxes to intuitive distance units.
Check the distance to a named star quickly without manual unit conversions.
Teach students the parsec definition and the parallax-distance relationship.
Verify hand-computed stellar distances against this tool.
Explore how measurement precision (mas vs. arcsec) affects distance accuracy.
Compare distances of nearby vs. distant stars on the same scale.
Estimate distance uncertainty from a parallax measurement error.
Convert between parsecs, light-years, and astronomical units in one step.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu