Parsec Converter

Convert star distances between parsecs, light years, and AU.

What it does

Stars sit in parsecs, light travel in light years, orbits in astronomical units. One parsec is the distance with one arcsecond of parallax, about 3.26 light years.

The tool above converts among pc, ly, and AU. The defaults convert 1 pc, so the page opens showing 3.26156 ly. Pick new units to update the result.

The meter anchors are exact: 1 AU is 149597870700 m, 1 light year is 9460730472580800 m (c times Julian year), 1 pc is 648000 over pi AU. Ratios follow by division.

Use AU inside systems, light years for messaging times, parsecs for catalogs. Proxima sits near 1.3 pc, or 4.25 light years.

This page shows the factors, one full hand checked example at 1 pc, a reference table, and practice checks that run through the tool.

How to use

1. Type the distance number.
2. Pick the starting unit.
3. Pick the target unit.
4. Read the main result.
5. Use pc for deep catalogs.
6. Use AU for orbits.

Formula

AU

149597870700 m exact.

ly

9460730472580800 m.

pc

648000/pi AU.

Light year uses Julian year 365.25 days with defined c. Parsec follows from AU and arcsecond geometry.

Worked example

Reference table

Limits

• Geometric units only. Redshift distances need cosmology.
• Julian year for light year here.
• Five decimals suit catalogs.
• Proper motion ignored.

Common mistakes

• Mixing light years with years of time.
• Using 3.26 rounded mid chain.
• Reading AU as light minutes (8.3167 per AU).
• Expecting parallax to equal distance without inversion.
• Comparing redshift z directly as pc.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• BIPM: SI Brochure
• Wikipedia: Julian day (astronomical dating)

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