Parsec

A parsec is an astronomy distance unit built from geometry: if an object is 1 parsec away, the radius of Earth’s orbit around the Sun (1 AU) would appear to span 1 arcsecond from that object. This comes from the parallax method, where nearby stars seem to shift slightly against distant background stars as Earth moves to opposite sides of its orbit. Because that angle is tiny, the unit turns out to be much larger than an astronomical unit and is convenient for measuring star distances.

Use this when converting between parsecs and light-years, or when working with stellar distances measured by parallax. It is especially common in astronomy, astrophysics, and telescope data where distances to stars, nebulae, and galaxies are reported in pc or kpc.

• Confusing parsecs with light-years and assuming they are the same unit
• Using the parsec relation for very small solar-system distances instead of astronomical units
• Forgetting that the conversion is approximate and rounding too early in multi-step calculations