Astronomical unit and light-year
The astronomical unit and light-year measure lengths, with different defining conventions.
Key knowledge
The astronomical unit, symbol au, is exactly 149597870700 metres; it is a fixed length, not the changing instantaneous Earth–Sun separation. A light-year is the distance light travels in vacuum in a year, approximately 9.46 × 10^15 metres. It measures length, not elapsed time; precise conversion must specify the year convention.
Conditions and scope
- The au value is the convention adopted by IAU in 2012 and recorded by BIPM; the light-year value here is rounded.
- For cosmological sources, choosing a length unit does not choose between luminosity, angular-diameter or other model-dependent distance measures.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected SI Brochure printed pp.140–141 Table 8 and note (j) for exact au, and NASA's light-year distance definition/rounded conversion. No exact Julian-year conversion, current object-distance catalogue or historical AU measurement claim is asserted.
2026-10-10 · Independent Mathematics and Information editor (/root/core_math_information). Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.
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