Light Year & Astronomical Distance Calculator
Convert astronomical distances between light years, parsecs, astronomical units, and kilometres. Calculate travel times at any spacecraft speed.
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Astronomical Distance Guide
What's the key thing to understand about Standard Distance Units?
Astronomical Unit (AU): mean Earth-Sun distance. 1 AU = 149,597,870 km ≈ 150 million km. Light travel time: 8 minutes 19 seconds. Used for distances within the solar system. Mars: 1.5 AU. Jupiter: 5.2 AU. Pluto: 39 AU. Voyager 1: ~165 AU and counting. Light Year (ly): distance light travels in one year. 1 ly = 9.461 × 10¹² km = 63,241 AU. Used for stellar distances. Light-second: 299,792 km. Light-minute: 18 million km. Light-hour: 1.08 billion km. Parsec (pc): defined from parallax. 1 parsec = 3.26 light-years, a unit astronomers prefer because it relates directly to the parallax angle measured from Earth, rather than requiring a separate conversion.
What's the key thing to understand about Familiar Astronomical Distances?
Moon: 1.28 light-seconds (384,000 km). Sun: 8.3 light-minutes. Mars (closest approach): 3.0 light-minutes. Jupiter: 35 light-minutes. Pluto: 5.3 light-hours. Voyager 1 (current): ~21 light-hours. Heliopause (edge of solar system): ~1 light-day. Nearest star Proxima Centauri: 4.24 light-years. Visible stars (naked eye): mostly 10-1000 ly. Centre of Milky Way: 26,000 ly. Edge of Milky Way disk: 50,000 ly. Andromeda Galaxy (M31): 2.5 million ly. Virgo Cluster: 65 million ly. Edge of the observable universe: approximately 46.5 billion light-years, which is farther than the universe's age would suggest due to the expansion of space itself during the time light has been travelling.
What do I need to know about Travel Times — Reality Check?
Voyager 1 speed: 17 km/s = 0.0057% of light speed. Years to reach Proxima Centauri at this speed: 74,000 years. Fastest spacecraft (Parker Solar Probe): 200 km/s = 0.067% of c. To Proxima Centauri: 6,300 years. Theoretical 10% of c spacecraft (light sail, fusion drive): 42 years to Proxima. Theoretical 99% of c: 4.3 years coordinated time. Less for crew due to time dilation. Reality: chemical rockets cannot reach significant fraction of c. Ion drives, light sails, and nuclear pulse propulsion are all theoretical concepts under active research that could someday reach a small fraction of light speed, but none currently exist at a scale capable of crewed interstellar travel.
What should I know about Looking Back in Time?
Light takes time to travel — looking at distant objects means looking back in time. Sun: see it as it was 8 minutes ago. If Sun vanished now, we'd notice in 8 minutes. Proxima Centauri: see it as it was 4.24 years ago. Andromeda Galaxy: see it as it was 2.5 million years ago. Most distant observable galaxies (z > 11): seen as they were 13+ billion years ago — light from when the universe was young. Cosmic Microwave Background: oldest light visible. From 380,000 years after the Big Bang. We cannot see further back than this, since the universe was opaque to light before this point, making the Cosmic Microwave Background the oldest observable light in existence.