Guide

Reading astronomical distances

Astronomical unit, light-year and parsec are three length units with very different step sizes. All three are defined by exact constants.

Three units, one quantity

All three describe a length, not a time. The astronomical unit is the mean Earth–Sun distance and measures exactly 149597870700 metres. The light-year is the distance light travels in vacuum in one Julian year of 365.25 days, exactly 9460730472580800 metres. The parsec is the distance at which the Earth's orbital radius subtends one arcsecond and measures about 3.0857 × 10¹⁶ metres.

Why the light-year is not a time

The name regularly causes confusion because it sounds like a duration. In fact it is a distance: the distance light travels in one year. When an object is said to be 100 light-years away, we see it as it was emitted 100 years ago — the distance is a length, the light travel time is the associated time.

When each unit is used

Within the solar system the astronomical unit is used because the numbers stay manageable: Jupiter orbits about 5.2 au from the Sun. For nearby stars the light-year is the customary figure because it translates directly into an observation time. For distances on galactic scales and beyond, the parsec is the professional standard because it links directly to parallax measurement.

Light-seconds as an intermediate step

For engineering calculations the light-second is practical because it directly states signal travel time. The Earth–Moon distance equals about 1.28 light-seconds, the Earth–Sun distance about 499 light-seconds or 8.3 light-minutes. That tells you how long a radio signal takes without any further conversion.

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Unit family: Length

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