Convert Light Years, AU, Parsecs & Miles
Last reviewed: April 2026
A light year distance calculator converts between light years, astronomical units, parsecs, kilometers, and miles. A light year is the distance light travels in one year — approximately 9.461 trillion kilometers — and is the standard unit for measuring interstellar distances.
Space is incomprehensibly vast, and everyday units break down at astronomical scales. A light year — the distance light travels in one year — is about 5.879 trillion miles. An astronomical unit (AU), the Earth-Sun distance, is 93 million miles. A parsec, used by professional astronomers, is 3.262 light years. This calculator converts between all five common distance units and contextualizes them with travel time estimates and reference distances to famous celestial objects. For converting between more everyday units, use our Unit Converter.
Each unit serves a different scale. Astronomical units measure distances within our solar system — Earth to Mars is about 1.5 AU, Earth to Pluto about 39 AU, and the Voyager 1 spacecraft is about 165 AU from the Sun. Light years are the most intuitive unit for interstellar distances: Proxima Centauri is 4.24 light years away, Sirius is 8.6, and the center of our Milky Way is about 26,000 light years distant. Parsecs are preferred in professional astronomy because they relate directly to the parallax measurement method — a star at 1 parsec shows exactly 1 arcsecond of parallax as Earth orbits the Sun. For speed-related calculations, try our Speed Calculator.
The travel time estimates highlight how unreachable the stars currently are. At the speed of a commercial jet (550 mph), reaching Proxima Centauri would take about 5.1 million years. The fastest spacecraft ever launched — the Parker Solar Probe at 430,000 mph — would still take about 6,500 years. Even at light speed (186,282 miles/second), which physics says is impossible for anything with mass, the trip takes 4.24 years. These distances are why interstellar travel remains in the realm of science fiction and why astronomers use remote observation for anything beyond our solar system. Calculate speeds and distances with our Speed of Sound Calculator.
| Object | Distance | Light Travel Time |
|---|---|---|
| Moon | 0.000000041 ly | 1.3 seconds |
| Sun | 0.0000158 ly | 8.3 minutes |
| Proxima Centauri (nearest star) | 4.24 ly | 4.24 years |
| Center of Milky Way | 26,000 ly | 26,000 years |
| Andromeda Galaxy | 2.537 million ly | 2.537 million years |
A light-year is the distance that light travels in one year through the vacuum of space: approximately 5.879 trillion miles (9.461 trillion kilometers). Despite containing the word "year," a light-year is a unit of distance, not time — a distinction that frequently confuses non-astronomers. Light moves at 299,792,458 meters per second (approximately 186,282 miles per second), and over the course of one Julian year (365.25 days), it covers a staggering distance. For reference, light from the Moon reaches Earth in about 1.3 seconds, sunlight takes about 8 minutes and 20 seconds, and light from the nearest star beyond the Sun (Proxima Centauri at 4.24 light-years) takes over four years. When astronomers observe Proxima Centauri, they see it as it was 4.24 years ago — they are literally looking back in time.
Alternative distance units used in astronomy include the astronomical unit (AU), which equals the average Earth-Sun distance (about 93 million miles or 150 million kilometers), and the parsec (pc), which equals approximately 3.26 light-years. Parsecs are based on parallax measurement: one parsec is the distance at which a star would show one arcsecond of apparent motion against the background stars as Earth orbits the Sun. Professional astronomers prefer parsecs because they relate directly to the observational technique used to measure stellar distances, but light-years are more intuitive for general audiences because they connect distance to the familiar concept of light travel time.
Within our solar system, distances are measured in AU. Mercury orbits at 0.39 AU, Earth at 1 AU, Jupiter at 5.2 AU, and Neptune at 30 AU. The Voyager 1 spacecraft, launched in 1977 and now the most distant human-made object, is approximately 164 AU from the Sun (as of 2025) — it has been traveling for over 47 years and has covered less than 0.003 light-years. At its current speed of about 38,000 mph, Voyager 1 would take approximately 73,000 years to reach Proxima Centauri if it were heading in that direction (it is not). This comparison illustrates the immense gulf between interplanetary distances (measurable in AU and reachable within human lifetimes) and interstellar distances (measured in light-years and currently unreachable).
The Milky Way galaxy spans approximately 100,000 light-years in diameter and contains an estimated 100-400 billion stars. Our solar system sits about 26,000 light-years from the galactic center. The nearest large galaxy, Andromeda, is 2.537 million light-years away — when we observe it through a telescope, we see light that left Andromeda before modern humans evolved. The observable universe extends approximately 46.5 billion light-years in every direction (larger than its 13.8-billion-year age would suggest because space itself has been expanding). Distances at this scale are measured in megaparsecs (Mpc), where 1 Mpc equals 3.26 million light-years. The Hubble constant (approximately 70 km/s/Mpc) describes how rapidly distant galaxies recede from us due to cosmic expansion.
Determining how far away celestial objects are is one of astronomy's greatest challenges, addressed through a "cosmic distance ladder" of increasingly indirect methods. For nearby stars (within about 1,000 light-years), stellar parallax measures the tiny apparent shift in a star's position as Earth orbits the Sun. The Gaia spacecraft has measured parallax distances for over 1.8 billion stars with unprecedented precision. Beyond parallax range, standard candles — objects with known luminosity — provide distance estimates. Cepheid variable stars pulsate with periods proportional to their intrinsic brightness, allowing astronomers to determine their true brightness from their pulsation period and then calculate distance from how dim they appear. Type Ia supernovae serve as standard candles for even greater distances, enabling measurements across billions of light-years and leading to the 1998 discovery that the expansion of the universe is accelerating.
The finite speed of light imposes hard limits on real-time communication across space. Controlling a Mars rover in real-time is impossible because radio signals (traveling at light speed) take 4-24 minutes to reach Mars depending on orbital positions — a 40-minute round trip at maximum distance means sending a "stop" command after seeing the rover approach a cliff arrives far too late. Instead, rovers are programmed with autonomous navigation routines and receive high-level instructions rather than moment-to-moment control. For missions to Jupiter (35-52 minutes one-way), Saturn (68-84 minutes), or beyond, even greater autonomy is required. This communication delay is not a technological limitation that better equipment can solve — it is a fundamental physical constraint that defines the boundary between remotely-operated and autonomous exploration.
See also: Unit Converter · Speed Calculator · Speed of Sound Calculator · Radioactive Decay Calculator · Scientific Notation Calculator
→ A light year is a distance, not a time. It's the distance light travels in one year: 9.461 × 10¹² km. Light moves at 299,792 km/s, so in one year it covers a staggering distance. When astronomers say a star is "10 light years away," they mean its light took 10 years to reach us — we see it as it was 10 years ago.
→ Parsecs are the professional astronomer's preferred unit. One parsec = 3.26 light years. It's based on stellar parallax — the apparent shift of a nearby star against distant background stars as Earth orbits the Sun. "Mega-parsec" (Mpc = 3.26 million ly) is standard for galactic and cosmological distances. Explore related physics with our Speed of Sound Calculator.
→ The nearest star system (Alpha Centauri) would take ~6,300 years to reach at current spacecraft speeds. Voyager 1, the fastest human-made object to leave the solar system, travels at ~38,000 mph. At that speed, reaching Proxima Centauri (4.24 ly) takes about 73,000 years. Even at 10% the speed of light, the trip takes 42 years.
→ Because light has a finite speed, looking far into space means looking back in time. The Andromeda Galaxy's light we see left 2.5 million years ago. The cosmic microwave background radiation shows the universe as it was 13.8 billion years ago. The "observable universe" has a radius of 46.5 billion light years due to cosmic expansion.
See also: Speed of Sound Calculator · Speed Converter · Unit Converter · Scientific Notation Calculator