What Is Stellar Parallax? Formula & Diagram
Stellar parallax is a nearby star’s apparent shift as Earth orbits the Sun. Learn d = 1/p and the parsec, with a parallax diagram and worked examples.
The idea behind it
As Earth moves around the Sun, a nearby star appears to shift against distant background stars. The annual parallax is half of the total angular shift between opposite points in Earth’s orbit.
The relationship to remember
1 / 0.1 arcseconds = 10 parsecs = 32.62 light-years.
Make your own discovery
Reduce the parallax angle by a factor of ten. The inferred distance increases by the same factor.
Open the Stellar Parallax Calculator & Diagram ↗Key formulas
| Quantity | Formula | Note or example |
|---|---|---|
| Parallax distance | d (parsecs) = 1 ÷ p (arcseconds) | p is the annual parallax |
| Parsec in light-years | 1 pc ≈ 3.26 ly | |
| Parsec in AU | 1 pc ≈ 206,265 AU |
Worked examples
Each row uses one of this lab’s presets and the same model as the interactive tool.
| Example | Distance | Light travel | Parallax |
|---|---|---|---|
| Proxima Centauri | 1.3 pc | 4.25 years | 0.768″ |
| Ten parsecs | 10 pc | 32.62 years | 0.1″ |
| One kiloparsec | 1,000 pc | 3,261.56 years | 0.001″ |
What this model leaves out
The standard small-angle parsec relation with exact input. The diagram exaggerates angles and compresses distances. Real measurements include uncertainty and corrections.
Check your understanding
What is a parsec?
A parsec is the distance corresponding to one arcsecond of annual parallax, approximately 3.26 light-years.
Why are distant stars harder to measure?
Their parallax angles are smaller, so a fixed angular uncertainty represents a larger fraction of the measurement.
What is stellar parallax?
Stellar parallax is the small apparent shift of a nearby star against much more distant stars as Earth orbits the Sun. The annual parallax angle p is half the total shift; 1 ÷ p gives the distance in parsecs.
Continue with the formula reference or test yourself in space practice.