Length units beyond the metre — light-years, ångströms, and nautical miles
Why some length units live in physics and astronomy, not in everyday conversation.
The SI defines exactly one length unit: the metre. Everything else is either a metric prefix (km, cm, mm), an old non-metric unit (mile, foot, inch), or a special-purpose unit designed for a specific context.
Three special-purpose length units
Light-year (ly): distance light travels in one Julian year. About 9.46 × 10¹⁵ metres, or roughly 63,000 AU, or about 0.31 parsecs. Used in astronomy for the distances to stars and galaxies. There are no everyday uses.
Nautical mile (nmi): 1,852 metres, originally defined as one minute of latitude along any meridian. The corresponding speed unit is the knot. Marine navigation and aviation both use nautical miles because they map cleanly to the latitude/longitude grid.
Ångström (Å): 10⁻¹⁰ metres (0.1 nm). Used in spectroscopy and crystallography — visible light wavelengths are on the order of 4,000–7,000 Å. Modern spectroscopy often uses nm, but published atomic tables often use Å.
Why these three over something else?
Three is the right answer because:
- Astronomers need a unit that doesn't make every day-count completely unwieldy. Saying "4 light-years to Proxima Centauri" beats "38,000,000,000,000,000 metres". - Navigators need a unit tied to the Earth coordinate grid so charts work directly. - Spectroscopists need a unit tied to the size of an atom so tables of atomic lines have integer-ish numbers.
Conversions
- 1 light-year = 9.4607 × 10¹⁵ m - 1 nautical mile = 1,852 m - 1 ångström = 10⁻¹⁰ m
The site lists all three plus the usual metric prefixes.