A parsec (symbol: pc) is an astronomical unit of distance used to measure how far away stars and other celestial objects are from Earth.
Definition of a Parsec
A parsec is defined as the distance at which a star would have an annual parallax angle of exactly 1 arcsecond (1").
In simpler terms:
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Because Earth orbits the Sun, nearby stars appear to shift slightly against the distant background stars. This apparent shift is called parallax.
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If a star’s parallax is measured to be 1 arcsecond, then the star is exactly 1 parsec away.
How far is a Parsec?
1 parsec is approximately:
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3.086 × 10¹⁶ meters,
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or about 3.26 light-years (the distance light travels in 3.26 years).
How is a Parsec calculated?
If pp is the star’s parallax angle in arcseconds, the distance dd in parsecs is given by the simple formula:
d=1pd = \frac{1}{p}
For example, if a star’s parallax is 0.5 arcseconds:
d=10.5=2 parsecsd = \frac{1}{0.5} = 2 \text{ parsecs}
Uses of the Parsec
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Measuring distances to stars, star clusters, and galaxies.
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Serving as a fundamental unit for mapping the structure of our galaxy and the universe.
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Calibrating other distance measurement methods in astronomy.
Parsec vs Light-Year
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A light-year is the distance light travels in one year.
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A parsec is based on the geometry of Earth’s orbit and the parallax angle.
Both units are common in astronomy, but parsecs are especially favored for precise stellar distance measurements.