How far is a light year? About 9.46 trillion kilometers, or 5.88 trillion miles, which is the distance light covers in one year through the vacuum of space. The exact figure is 9,460,730,472,580.8 km, set by the International Astronomical Union.
That is roughly 63,241 times the distance between Earth and the Sun. Even so, it is a small step on the cosmic scale: the nearest star beyond the Sun is more than four of them away.
The exact definition, and where the number comes from
The IAU defines the light year as the distance light travels in a vacuum in one Julian year. A Julian year is exactly 365.25 days, the year the IAU uses unless another is specified, so the unit is fixed and needs no leap-year adjustment.
The other ingredient is the speed of light. NIST lists it as 299,792,458 meters per second, an exact value because the meter itself is defined from it.
Multiply the two and you get the whole answer. 365.25 days hold 31,557,600 seconds, and 31,557,600 times 299,792,458 m gives 9,460,730,472,580,800 meters.
Convert that with 1.609344 km per mile and you get about 5,878,625,373,184 miles. NASA rounds it the same way on its light-year explainer: 186,000 miles per second, 5.88 trillion miles per year.
- Kilometers: 9,460,730,472,580.8 (exact, IAU)
- Miles: about 5.88 trillion
- Astronomical units: about 63,241
- Parsecs: about 0.31
Video: Conceptual Academy
How long light takes to reach the Moon, the Sun and beyond
Inside the solar system a light year is far too big to be useful, so NASA JPL counts in light-time instead.
The Moon sits an average of 384,400 km away, so by our arithmetic light crosses that gap in about 1.3 seconds. JPL lists about 3 seconds for the round trip.
Sunlight needs about 8.3 minutes to cover 1 au, the 149.6 million km between the Sun and Earth. JPL also notes that light travels roughly a billion kilometers in an hour, about 1.5 times the Sun-Jupiter distance.
That is why sunlight glinting off Saturn’s rings has been on the road for more than an hour before it reaches your telescope. Out past Neptune, where most known dwarf planets orbit, light-time stretches to several hours.
The Sun’s own reach nearly touches one light year. JPL describes the Oort cloud, the reservoir thought to feed long-period comets, at roughly 50,000 au from the Sun, which it calls nearly a light year.
| Destination | Distance | Time for light to arrive |
|---|---|---|
| The Moon | 384,400 km (238,855 mi), average | About 1.3 seconds |
| The Sun | 1 au, about 149.6 million km | About 8.3 minutes |
| Proxima Centauri, nearest star | 4.25 light years, about 40 trillion km | About 4.25 years |
| Across the Milky Way | About 100,000 light years | About 100,000 years |
| Andromeda Galaxy (M31) | 2.5 million light years, about 24 quintillion km | About 2.5 million years |
Source: NASA (Moon facts, JPL Basics of Space Flight, Imagine the Universe nearest star, Milky Way, Hubble Messier 31), opened October 2026. Kilometer conversions and the Moon light-time are our arithmetic from the cited values.
The light you see from it on a dark night left the galaxy 2.5 million years ago, so you are looking at Andromeda as it was then, not as it is now.
A light year measures distance, not time
A light year is a distance, so it tells you how far, never how long. Saying a trip took “three light years” is like saying a drive took three miles.
Light covers one light year in one year, so for light the two numbers match. For anything slower, they split apart fast.
NASA pages do not always agree on Proxima Centauri. Imagine the Universe gives 4.25 light years, while JPL’s training text rounds it to 4.2.
Neither is wrong, they just round differently. Check which figure a source uses before you compare two articles.
Light year vs parsec: which one astronomers actually use
Professional astronomers mostly reach for the parsec. NASA defines it as the distance at which 1 au spans an angle of one arcsecond, and puts it at 3.26 light years.
The IAU gives the parsec as about 30.857 trillion km, or about 206,000 au. It also notes that the light year shows up mostly in popular writing, which is why you meet it in news stories and the parsec in research papers.
The parsec wins in research because it falls straight out of parallax, the tiny shift in a star’s position as Earth orbits the Sun. For galaxy-scale work, astronomers add prefixes: a kiloparsec is 1,000 parsecs and a megaparsec is a million.
On that scale, NASA puts the Milky Way at about 30 kiloparsecs across, roughly 100,000 light years, with the Sun sitting about 8 kiloparsecs from the center.
How long it would take you to travel one light year
Light needs one year. Voyager 1, which NASA says is leaving the Sun at about 17.3 km per second, would need more than 73,000 years to reach Proxima Centauri.
Divide one light year by Voyager’s speed and you get roughly 17,300 years for a single light year. NASA’s exoplanet team puts it another way: a jet flight to Proxima Centauri would take about 5 million years.
JPL’s light-time table puts Voyager 1 about 173 au out, a full light-day, so a command sent to it today arrives tomorrow. Mars, with its two small moons, Phobos and Deimos, is far closer, yet its signals still take minutes, not the Moon’s 1.3 seconds.
Frequently asked questions
How many miles is one light year?
One light year is about 5.88 trillion miles. The exact metric figure is 9,460,730,472,580.8 km, which works out to roughly 5,878,625,373,184 miles. NASA rounds it to 5.88 trillion miles, with light moving at about 186,000 miles per second.
Is a light year a measure of time?
No. A light year is a unit of distance: how far light travels in one Julian year of 365.25 days. Only light itself takes exactly one year to cover it. That is also why Andromeda’s light, 2.5 million light years away, left the galaxy 2.5 million years ago.
How many light years away is the nearest star?
Proxima Centauri, the nearest star to the Sun, is about 4.25 light years away according to NASA. Alpha Centauri A and B sit a little farther, at about 4.35 light years. Light from Proxima takes about 4.25 years to arrive, while Voyager 1 would need more than 73,000.
