As Earth orbits the Sun, nearby stars shift slightly against distant ones. This parallax angle is tiny – Proxima Centauri’s is under one arcsecond – but measuring it gives the most direct distances in astronomy. The Gaia satellite has measured it for over a billion stars.
How to use the Parallax Calculator
- Find
- Star
- Parallax angle p
- Distance
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose a star or enter the parallax in arcseconds (or milliarcseconds), or switch to find the parallax of a given distance.
Inputs explained
| Field | What to enter |
|---|---|
| Find | Choose one: Distance from parallax, Parallax from distance |
| Star | Choose one: Proxima Centauri, Alpha Centauri A, Barnard’s Star, Sirius … |
| Parallax angle p | Units: arcsec, mas, ° |
| Distance | Units: pc, ly, au |
What each result means
- Distance = 1÷[p]
- Parallax angle = [p]
- In milliarcseconds = [p]×1000
Example values
How the main result changes when Parallax angle p changes and everything else stays at the example values above:
| Parallax angle p (arcsec) | Distance (pc) |
|---|---|
| 0.05 | 1.3012 |
| 0.075 | 1.3012 |
| 0.1 | 1.3012 |
| 0.125 | 1.3012 |
| 0.15 | 1.3012 |
| 0.2 | 1.3012 |
Formula
d (pc) = 1 / p (arcsec)
Worked example
Inputs — Find: Distance from parallax; Star: Proxima Centauri.
Results — Distance = 1.301236 pc; In milliarcseconds = 768.5 mas.
Key terms
- Arcsecond
- 1/3600 of a degree.
Frequently asked questions
What is a parsec?
The distance at which the parallax is 1 arcsecond (3.26 ly).
Why can't parallax measure distant galaxies?
The angle becomes too small to measure.
Which star has the largest parallax?
Proxima Centauri, about 0.768″.
How accurate is Gaia?
Down to about 10–20 microarcseconds for bright stars.