Table of Contents
What Is Parallax?
Parallax is the apparent shift in the position of a nearby object against a distant background when observed from two different positions. In astronomy, stellar parallax uses Earth's orbit as a baseline: a nearby star appears to shift slightly against background stars when observed six months apart. This is the most direct method for measuring stellar distances and is the basis of the parsec distance unit.
A parsec is defined as the distance at which an object has a parallax angle of exactly one arcsecond using a baseline of one astronomical unit (Earth-Sun distance). One parsec equals approximately 3.26 light-years or 3.086 × 10¹³ km. The nearest star, Proxima Centauri, has a parallax of 0.77 arcseconds, placing it 1.30 parsecs away.
Formula
Space Parallax Missions
| Mission | Precision | Range |
|---|---|---|
| Hipparcos (1989) | ~1 mas | ~500 pc |
| Gaia (2013) | ~0.02 mas | ~10,000 pc |
| Ground-based | ~50 mas | ~20 pc |
Frequently Asked Questions
Why is parallax limited in range?
As distance increases, the parallax angle decreases proportionally, eventually becoming too small to measure accurately. Ground-based telescopes can measure parallax to about 0.01 arcseconds (100 pc). Space missions like Gaia push this to micro-arcsecond precision, reaching thousands of parsecs.
What is a parsec?
A parsec (parallax arcsecond) is the distance at which 1 AU subtends an angle of 1 arcsecond. It equals 3.0857 × 10¹⁶ m, or about 3.26 light-years. Astronomers prefer parsecs because distance in parsecs is simply 1/parallax-in-arcseconds.