Cosmic Distance Converter
Convert between light-years, parsecs, astronomical units, kilometres, and more — with real landmark distances, light travel time, and scale comparisons that bring the universe to life.
Real measured distances to famous objects across the cosmos — instantly available in any unit.
| Object | Type | Distance | Light Travel Time | Notes |
|---|
*Distances are best current measurements and may vary slightly across sources. Light-years are the primary measurement, other units derived.
How long does light take to travel a given distance? Or how far does light travel in a given time?
Every astronomical distance unit, its exact value in metres, and how it’s defined and used.
| Unit | Symbol | Exact Value (km) | Definition & Use |
|---|---|---|---|
| Metre | m | 0.001 | SI base unit. Distance light travels in 1/299,792,458 seconds. Used for planetary surfaces, satellites. |
| Kilometre | km | 1 | 1,000 metres. Standard unit for Earth distances, also used in solar system measurements. |
| Mile | mi | 1.609344 | 1.60934 km. Used in US/UK contexts, historical astronomical records. |
| Astronomical Unit | AU | 149,597,870.7 | Average Earth–Sun distance. IAU defined as exactly 149,597,870,700 m. Used for solar system scales. |
| Light-Second | ls | 299,792.458 | Distance light travels in 1 second. Moon is ~1.28 light-seconds away. |
| Light-Minute | lm | 17,987,547.48 | Distance light travels in 1 minute. Sun is ~8.3 light-minutes from Earth. |
| Light-Year | ly | 9,460,730,472,580.8 | Distance light travels in one Julian year (365.25 days). Most common unit for interstellar distances. |
| Parsec | pc | 30,856,775,814,671.9 | Distance at which 1 AU subtends 1 arcsecond of parallax. = 3.26156 ly. Professional astronomy standard. |
| Kiloparsec | kpc | 3.086 × 10¹⁶ | 1,000 parsecs. Used for distances within the Milky Way galaxy. |
| Megaparsec | Mpc | 3.086 × 10¹⁹ | 1,000,000 parsecs. Standard unit for measuring distances between galaxies and galaxy clusters. |
| Gigalight-Year | Gly | 9.461 × 10²¹ | 1 billion light-years. Used for cosmic-scale distances and cosmological models. |
| Gigaparsec | Gpc | 3.086 × 10²² | 1 billion parsecs. Used in cosmology for describing the scale of the observable universe. |
Why Cosmic Distances Need Special Units
The observable universe spans approximately 93 billion light-years in diameter. The nearest star system to our own, Alpha Centauri, is roughly 41 trillion kilometres away. These numbers, when written out in kilometres or miles, become so unwieldy that they lose all intuitive meaning. A string of 13 digits says nothing useful about the overwhelming scale of the cosmos.
This is exactly why astronomers developed specialised units — the light-year, the parsec, and the astronomical unit — each calibrated to the scale of the measurement being made. Just as you would not measure the distance between cities in millimetres, you would not express the distance to the Andromeda Galaxy in kilometres. The right unit for the right scale is the foundation of clear scientific communication.
This converter handles all nine major distance units used in astronomy, from metres to gigaparsecs, with exact conversion factors defined to IAU (International Astronomical Union) standards.
The Light-Year: Not a Measure of Time
The most common misconception about the light-year is that it measures time. It does not. A light-year is a unit of distance — specifically, the distance that light travels through a vacuum in one Julian year (365.25 days). Given that light travels at exactly 299,792.458 kilometres per second, one light-year works out to precisely 9,460,730,472,580.8 kilometres — approximately 9.46 × 10¹² km, or just under 10 trillion kilometres.
The reason light-years are used rather than parsecs in popular science is purely communicative: “the star is 4 light-years away” immediately tells a general audience that the light we see from that star left 4 years ago, which connects distance to time in a way that is intellectually vivid. Professional astronomers almost universally prefer parsecs because they arise naturally from the parallax measurement technique — the primary method for determining stellar distances.
💡 Key insight: When you observe a star 100 light-years away, you are not seeing it as it is now. You are seeing it as it was 100 years ago — the light now reaching your eye left that star before you were born. Every telescope is a time machine looking into the past, and the distances in light-years tell you exactly how far into the past you are looking.
The Parsec: The Professional’s Unit
The parsec (symbol: pc) is the unit of distance most commonly used by professional astronomers. It is defined geometrically: one parsec is the distance at which one astronomical unit (the Earth–Sun distance) subtends an angle of exactly one arcsecond (1/3600 of a degree) as seen from a distant observer.
This definition connects directly to the parallax method of measuring stellar distances. When Earth moves from one side of its orbit to the other (a distance of 2 AU), nearby stars appear to shift slightly against the background of more distant stars — a phenomenon called parallax. A star at exactly 1 parsec distance would show a parallax angle of exactly 1 arcsecond. A star at 2 parsecs shows half an arcsecond. The inverse relationship is simple: distance in parsecs = 1 ÷ parallax angle in arcseconds.
One parsec = 3.26156 light-years = 206,264.8 AU = 3.086 × 10¹³ km. The Milky Way galaxy is about 30 kiloparsecs (kpc) in diameter. The distance to the Andromeda galaxy is approximately 778 kiloparsecs (0.778 Mpc).
The Astronomical Unit: The Solar System’s Ruler
The astronomical unit (AU) is the baseline measure for distances within our solar system. Since 2012, it has been defined by the IAU as exactly 149,597,870,700 metres (approximately 149.6 million km or 93 million miles). This corresponds closely to the average Earth–Sun distance, though the exact value was updated from the original observational definition to this precise integer.
Earth–Sun
1.000 AU = 149.6M km = 8.317 light-minutes. The definition of the AU.
Earth–Jupiter
~5.2 AU at closest approach. Jupiter’s distance varies as both planets orbit the Sun.
Earth–Saturn
~8.5–10.4 AU depending on orbital positions. Light takes 71–86 minutes.
Earth–Neptune
~30.1 AU average. The most distant planet. Light takes ~4.2 hours to reach it.
Voyager 1
~163 AU (as of mid-2026) — the most distant human-made object. Still in contact with NASA.
Oort Cloud
Estimated 2,000–100,000 AU. The hypothetical outer boundary of the solar system.
Conversion Factors: Exact Values
| From | To Light-Year | To Parsec | To AU | To Kilometre |
|---|---|---|---|---|
| 1 Light-Year | 1 | 0.30660 | 63,241.1 | 9.461 × 10¹² |
| 1 Parsec | 3.26156 | 1 | 206,264.8 | 3.086 × 10¹³ |
| 1 AU | 1.581 × 10⁻⁵ | 4.848 × 10⁻⁶ | 1 | 149,597,870.7 |
| 1 Kilometre | 1.057 × 10⁻¹³ | 3.241 × 10⁻¹⁴ | 6.685 × 10⁻⁹ | 1 |
| 1 Megaparsec | 3,261,564 | 1,000,000 | 2.063 × 10¹¹ | 3.086 × 10¹⁹ |
Notable Cosmic Distances: A Sense of Scale
Numbers alone rarely convey the true scale of the universe. Context and comparison are essential. Here is a hierarchy of cosmic distances, from the nearest to the most distant things we can observe:
- Moon: 384,400 km — about 1.28 light-seconds. Radio signals take 2.56 seconds round trip, which is why lunar astronauts experienced a perceptible delay in conversations with Earth.
- Sun: 1 AU = 149.6 million km — about 8.317 light-minutes. The light warming your skin right now left the Sun slightly over 8 minutes ago.
- Proxima Centauri: 4.2421 light-years — the closest known star to our Sun. At the fastest speed humans have ever sent a spacecraft (Helios 2, ~252,000 km/h), the journey would take approximately 18,000 years.
- Galactic Centre: ~26,000 light-years — the supermassive black hole (Sagittarius A*, mass ~4 million solar masses) at the heart of the Milky Way. The Event Horizon Telescope imaged it in 2022.
- Milky Way diameter: ~100,000 light-years (recent estimates suggest up to 200,000 ly including the outer disc).
- Andromeda Galaxy (M31): ~2.537 million light-years. The most distant object visible to the naked eye and the nearest major galaxy. It is approaching us at ~110 km/s and will merge with the Milky Way in approximately 4.5 billion years.
- Virgo Cluster: ~53.8 million light-years. The nearest major galaxy cluster, containing over 1,300 galaxies including M87, which hosts a supermassive black hole famously imaged by the Event Horizon Telescope in 2019.
- Observable universe radius: ~46.5 billion light-years (comoving distance). The cosmic microwave background radiation we detect today was emitted 380,000 years after the Big Bang, when the universe was only about 42 million light-years in radius. Space itself has expanded since then.
The Expanding Universe and Lookback Time
One subtlety that our converter deliberately handles is the distinction between a light-year as a distance and a light-year as a time. When astronomers say an object is 10 billion light-years away, they typically mean its comoving distance — the distance accounting for the expansion of the universe since the light was emitted. The lookback time — how long ago the light left — may be a different number, because space was smaller when the light departed.
For practical purposes involving interstellar (not intergalactic) distances, this distinction is negligible. The conversion factors in this tool use the standard light-year, parsec, and AU definitions without cosmological expansion corrections — which is appropriate for solar system, stellar, and nearby galactic calculations. For cosmological calculations involving redshifts above z ≈ 0.1, specialist tools incorporating Hubble’s law are needed.
How Distance Is Measured in Astronomy
The methods astronomers use to measure cosmic distances form what is called the cosmic distance ladder — a hierarchy of techniques, each calibrated against and extending the previous one:
- Radar ranging (solar system): Radio pulses bounced off planets and moons give precise distances to light-second precision within the solar system.
- Stellar parallax (up to ~10,000 ly): The apparent shift of nearby stars against background stars as Earth orbits the Sun. The Gaia space telescope has measured parallax for over 1.4 billion stars with extraordinary precision.
- Standard candles — Cepheid variables (up to ~100 Mly): Cepheid variable stars pulsate with a period directly related to their intrinsic luminosity. By comparing intrinsic and apparent brightness, distance is calculated. Used to calibrate the extragalactic distance scale.
- Type Ia supernovae (up to ~10 Gly): These explosions have near-constant intrinsic luminosity (“standard candles”). They were used in the 1998 discovery that the universe’s expansion is accelerating — a discovery earning the 2011 Nobel Prize in Physics.
- Cosmic microwave background / Hubble’s Law (cosmological scales): The recession velocity of distant galaxies (measured from redshift) combined with the Hubble constant gives distances across the observable universe.
Frequently Asked Questions
Exactly 9,460,730,472,580.8 kilometres — approximately 9.46 trillion km. This is calculated from the speed of light (299,792.458 km/s) multiplied by the number of seconds in one Julian year (365.25 × 24 × 60 × 60 = 31,557,600 seconds). In scientific notation: 9.461 × 10¹² km.
Both are units of astronomical distance, but defined differently. A light-year is the distance light travels in one year — a time-based definition. A parsec is defined geometrically: the distance at which one AU subtends one arcsecond of parallax angle. 1 parsec = 3.26156 light-years. Parsecs are the preferred unit in professional astronomy because they arise directly from the parallax measurement method. Light-years are more common in popular science because of their intuitive time-connection.
At the speed of the fastest human-made spacecraft ever launched (Parker Solar Probe, ~690,000 km/h at perihelion), travelling one light-year (9.461 × 10¹² km) would take approximately 1,561 years. At Voyager 1’s current speed (~61,000 km/h), it would take about 17,640 years to travel one light-year. Even at 10% of the speed of light — far beyond any current technology — a journey to our nearest star, Proxima Centauri (4.24 ly), would take about 42 years.
No. The observable universe is the spherical region from which light has had time to reach us since the Big Bang (~13.8 billion years ago). Its radius is about 46.5 billion light-years (not 13.8 billion, because space has expanded during the journey). The actual universe is almost certainly much larger — current models suggest it may be at least 250 times larger in volume than the observable portion, possibly infinite. We simply cannot see beyond the observable boundary because light from those regions has not reached us yet.
The Andromeda Galaxy (M31) is approximately 2.537 million light-years away, yet it is faintly visible to the naked eye under dark skies because it contains roughly 1 trillion stars — about twice the number in the Milky Way. Its combined luminosity is enormous. What you see as a faint smudge is actually the integrated light of a trillion suns. It is the most distant object visible to the unaided human eye and appears as a 3.44° wide oval patch — about 7 times the width of a full moon, though far fainter.
One Astronomical Unit (AU) = 149,597,870.7 km — roughly 150 million km. In everyday terms: if you could drive to the Sun at 100 km/h without stopping, the journey would take over 170 years. A commercial aeroplane at 900 km/h would take about 19 years. Light, however, covers this distance in just 8 minutes and 20 seconds — which is why it’s called “8 light-minutes.”
