光年

• Reading: こうねん
• Romaji: konen
• Kanji breakdown: 光 (ko) — light; 年 (nen) — year
• Pronunciation: /koː.neɴ/

Meaning

Light year; the distance that light travels in one year in a vacuum, approximately 9.461 × 10¹⁵ metres. Used as a unit of astronomical distance.

Despite containing 年 (year), 光年 is a unit of distance, not time. It is used in astronomy to express the vast distances between stars and galaxies. The nearest star system to our sun, Alpha Centauri (アルファ・ケンタウリ), is about 4.37 light years away. In popular science writing and everyday speech, 光年 is also used figuratively to mean an extremely large gap or difference: 技術的には光年の差がある (there is a light year of difference in technology). The related term 光秒 (light second) is used for smaller astronomical distances.

Examples

1. アンドロメダ銀河は地球から約250万光年離れており、肉眼で見える最も遠い天体の一つだ。 The Andromeda Galaxy is approximately 2.5 million light years from Earth, making it one of the most distant objects visible to the naked eye.
2. 光年は時間の単位ではなく、光が1年間に進む距離を示す長さの単位である。 A light year is not a unit of time but a unit of length indicating the distance light travels in one year.
3. 現在の宇宙探査技術では、数光年先の星系に到達するのは事実上不可能に近い。 With current space exploration technology, reaching a star system several light years away is practically impossible.

Usage Guide

Context: astronomy, astrophysics, science journalism, everyday speech (figurative)

Tone: technical to neutral

Origin & History

Modern Sino-Japanese compound: 光 (ko) — light + 年 (nen) — year. A calque of the English/German 'light year' (Lichtjahr). Adopted in Meiji-era Japan as part of the introduction of modern astronomy, where 光 (light) and 年 (year) combine to form the compound for the astronomical unit of distance.

Cultural Context

Era: Meiji-Modern

Generation: All ages

Social background: Universal