Parallax Calculator
Calculate stellar distances using the parallax method. Enter milliarcsecond angle to get distance in parsecs, light-years, AU, and km with interactive charts.
About This Calculator
The Parallax Calculator computes the distance to nearby stars using the stellar parallax method — one of the most fundamental techniques in astronomy. By measuring the apparent shift in a star's position as Earth orbits the Sun, astronomers can determine how far away that star is. This calculator makes it easy for students, educators, and astronomy enthusiasts to explore stellar distances.
The formula is simple: distance in parsecs (pc) equals 1 divided by the parallax angle in arcseconds: D = 1 / P. Since parallax angles are extremely small (fractions of an arcsecond), they are typically expressed in milliarcseconds (mas), where 1 mas = 0.001 arcseconds. The calculator also converts the distance to light-years (1 pc = 3.26 ly), astronomical units (1 pc = 206,265 AU), and kilometers (1 pc = 3.086 × 10¹³ km). Choose from popular preset stars like Proxima Centauri, Sirius, Vega, and Betelgeuse, or enter a custom parallax angle.
How Stellar Parallax Works
As Earth travels from one side of its orbit to the opposite side six months later, our viewing position changes by about 2 astronomical units (300 million kilometers). A nearby star appears to shift against the distant background stars. Half of this angular shift is the parallax angle. The method works for stars up to about 100 parsecs (326 light-years) away — beyond that, the angle becomes too small to measure precisely even with modern instruments.
About the Data
Preset star parallax values come from the European Space Agency's Hipparcos satellite catalog and Gaia mission data. These represent the best available measurements and are updated as new data becomes available. The calculator uses the standard IAU-defined conversion factors: 1 parsec = 3.261563777 light-years = 206,264.806 AU = 3.085677581 × 10¹³ km.
Frequently Asked Questions
What is stellar parallax?
Stellar parallax is the apparent shift in position of a nearby star against distant background stars due to Earth's orbit around the Sun. Astronomers measure this shift as an angle to calculate the star's distance.
How do you calculate distance using parallax?
The distance in parsecs is calculated as D = 1 / P, where P is the parallax angle in arcseconds. Convert milliarcseconds (mas) to arcseconds by dividing by 1000. For example, Sirius with 379.2 mas parallax gives 1 / 0.3792 = 2.64 parsecs.
What is a parsec?
A parsec (pc) is the distance at which an object has a parallax angle of 1 arcsecond. One parsec equals approximately 3.26 light-years, 206,265 astronomical units, or 3.086 × 10¹³ kilometers.
What is the closest star to Earth?
The closest star to Earth is Proxima Centauri, with a parallax of 768.5 milliarcseconds, placing it about 1.30 parsecs or 4.25 light-years away.
Can parallax measure distances to all stars?
No, stellar parallax method is limited to stars within about 100 parsecs (326 light-years). Beyond that, the parallax angle becomes too small to measure accurately with current technology. ESA's Gaia mission measures parallax for over a billion stars with unprecedented precision.
Who discovered stellar parallax?
The first successful measurement of stellar parallax was made by Friedrich Bessel in 1838 for the star 61 Cygni. This finally confirmed Copernicus's heliocentric model of the solar system.
How is parallax used in astronomy besides distance measurement?
Parallax is fundamental to the cosmic distance ladder, calibrating standard candles like Cepheid variables and Type Ia supernovae. It also helps determine stellar luminosities, masses from binary orbits, and the scale of the universe.
What is the Hipparcos mission?
The European Space Agency's Hipparcos satellite (1989-1993) measured parallax for over 100,000 stars with accuracy up to 1 milliarcsecond. Its successor Gaia (2013-present) measures over a billion stars with precision down to 0.01 milliarcseconds.