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A parsec was an astronomical measure of length that stood for "parallax arc-second". It was based on the Earth method of measuring interstellar distances using trigonometry. A star located one parsec away from an observer would have a "parallax angle" of one arc-second.

Based on this, one parsec was exactly equal to 360×60×60 / 2π au (206,264.8 au or ∼3.26 light years).

A parsec is more exactly defined than a light year, since the length of the latter is dependent on the length of the year being used. For example, the IAU uses a Julian year of 365.25 days, while other sources may use a Gregorian year of 365.2425 days, or even some other definition of year.

## Comparative list of lengths

Each of the following gives a conversion to IAU light years, with appropriate use of significant digits.
2.2 milliparsecs ≈ 0.0072 light years, or 455.326 au.
1/16 parsec ≈ 0.2 light years.
0.5 parsec ≈ 1.6 light years.
0.54 parsecs ≈ 1.76 light years.
1 parsec ≈ 3.2 light years.
1.4 parsecs ≈ 4.6 light years.
2.3 parsecs ≈ 7.5 light years.
2.72 parsecs ≈ 8.87 light years.
5 parsecs ≈ 16.308 light years.
20 parsecs ≈ 65.231 light years.
22.3 parsecs ≈ 72.7 light years.
23 parsecs ≈ 75 light years.
100 parsecs ≈ 326 light years.
• The Metrons hurled the Enterprise 500 parsecs distant from their previous location. (TOS: "Arena")
500 parsecs ≈ 1.63 thousand light years.
5,000 parsecs ≈ 16.308 thousand light years.