Light-Year Converter

A light-year is the distance light travels in one year through a vacuum — approximately 9.461 trillion kilometres (9.461 × 10¹² km). It is not a unit of time, despite the word "year" in its name. Light travels at 299,792.458 km per second (exactly, by definition since 1983 when the metre was redefined in terms of the speed of light). This means in one second, light covers the distance from London to New York and back about 37 times. A light-year spans a distance so vast that it makes even the scale of the Solar System — which light crosses in about 5.5 hours at its widest — feel tiny.

Astronomers use several distance units depending on scale. The Astronomical Unit (AU) — the average Earth-Sun distance, about 149.6 million km — is used within the Solar System: Neptune is 30 AU away, the Voyager 1 probe (the most distant human-made object) is around 160 AU. For interstellar distances, the light-year and the parsec (about 3.26 light-years, defined as the distance at which one AU subtends one arcsecond of angle) are used. The parsec is the professional astronomer's preferred unit because it emerges naturally from the parallax method of measuring stellar distances — the foundation of cosmic distance measurement.

The nearest star system, Alpha Centauri (including Proxima Centauri), is 4.24 light-years away. The Milky Way galaxy is approximately 100,000 light-years in diameter. The Andromeda Galaxy — the most distant object visible to the naked eye — is 2.537 million light-years away, meaning the light we see left Andromeda when our ancestors were making stone tools. The observable universe extends to about 46 billion light-years in every direction (larger than 13.8 billion light-years because of cosmic expansion). These numbers are genuinely incomprehensible at human scales, which is why unit conversion tools like this one are valuable for grounding them in familiar distances.