Astronomy
How Astronomers Measure Distances Using Parallax
Quick fact
The nearest star, Proxima Centauri, has a parallax of about 0.77 arcseconds, corresponding to a distance of about 4.24 light-years. This is among the largest parallax angles measurable.
Why this is interesting
Hold your finger in front of your nose and close one eye, then switch eyes. Your finger appears to jump against the background. Astronomers use this same principle to measure the distances to stars.
Read the full explanation
Understanding How Astronomers Measure Distances Using Parallax
Imagine Earth’s orbit as a giant baseline. When we look at a nearby star in January, and again six months later (when Earth is on the opposite side of its orbit), the star appears to shift relative to the distant background stars. This apparent shift is the parallax angle. Using simple geometry, if you know the angle and the length of the baseline (the diameter of Earth’s orbit, about 2 astronomical units), you can calculate the distance to the star. The effect is tiny—only a fraction of an arcsecond even for the closest stars—which is why it wasn't measured until the 19th century.
A deeper explanation
The parallax method relies on the fact that Earth moves around the Sun. Astronomers measure the parallax angle p in arcseconds. The distance d in parsecs is given by d = 1/p. One parsec is the distance at which a star would have a parallax of 1 arcsecond, which is about 3.26 light-years. This formula works because the baseline is fixed (1 astronomical unit from the Sun to Earth, but the full baseline of 2 AU gives the angle; the parallax method defines the parsec such that the Sun–Earth distance is the baseline). The limitation is that for stars farther than about 100 parsecs, the angle becomes too small to measure accurately with current technology, requiring other methods like spectroscopic parallax or standard candles. The European Space Agency’s Gaia mission is currently measuring parallaxes for over a billion stars with unprecedented precision, refining our understanding of the galaxy’s structure.