How far is a light-year?
A light-year is the distance light travels in a year: about 9.5 trillion km. Pick anything from the Moon to the Andromeda Galaxy, or type any distance, and see how long light and our fastest spacecraft take to cross it, when the light you see left, and how big it would be in a model where the Sun is a grain of sand.
1,350 light-years away.
That’s 12.8 quadrillion km (7.9 quadrillion miles), or 85.4 million times the Earth–Sun distance, or 413.9 parsecs.
The light you see from it tonight left around the year 676.
Shrink the Sun to a grain of sand 1 mm wide, and this distance becomes 9,171 km.
- Walking291.4 billion years
- A car on the highway14.6 billion years
- An airliner1.6 billion years
- Voyager 1, leaving the Solar System24 million years
- Parker Solar Probe at its fastest (2024)2.1 million years
- Light1,350 years
Why do astronomers use light-years?
Because kilometers run out of room. The nearest star after the Sun is 40.2 trillion km away; the Andromeda Galaxy is about 2.5 million light-years, some 24 billion billion kilometers. A light-year packs that into a unit people can say, and it carries a second meaning for free: it tells you how old the light is. Look at the Orion Nebula tonight and you see it as it was about 1,350 years ago.
Light is the fastest thing there is, 299,792 km every second. Even so it takes 1.3 seconds to cross from the Moon, 8 minutes from the Sun and 72 minutes from Saturn tonight. Voyager 1, launched in 1977 and now leaving the Solar System at about 17 km a second, would need 75,000 years to reach the nearest star.
How big is the universe on a scale you can picture?
Shrink the Sun to a grain of sand 1 mm wide. The Earth is then a speck 106 mm away, Saturn sits about 1 m out, and the nearest star, Proxima Centauri, is another grain of sand 29 km away. That emptiness is why, when two galaxies collide, their stars almost never hit each other. You can watch one happen in a galaxy collision simulator in your browser.
Questions people ask
How far is a light-year in km and miles? A light-year is the distance light travels in one year: 9.5 trillion km (about 5.9 trillion miles). It's a distance, not a length of time.
How long would it take to travel one light-year? Light takes a year, by definition. Voyager 1, one of the fastest spacecraft leaving the Solar System at about 17 km a second, would take 18,000 years. A car at highway speed would take 11 million years.
How far away is the nearest star? Apart from the Sun, the nearest star is Proxima Centauri, 4.2465 light-years away. Its light takes about four and a quarter years to reach us; Voyager 1 would need 75,000 years.
How many light-minutes away is the Sun? Today the Sun is 148.2 million km away, so its light takes 8 minutes to reach us. The Moon is 1.3 seconds away, and Saturn 72 minutes (November 7, 2026).
What is a parsec? About 3.26 light-years: the distance at which a star would appear to shift by one arcsecond as the Earth goes from one side of its orbit to the other. Astronomers use parsecs because they come straight from that measured shift, the parallax.
Sources
- Light-year: the distance light travels in one Julian year (the IAU's definition, summarized with references)
- NASA NSSDC: Sun fact sheet
- ESA Gaia Data Release 3 (Proxima Centauri's parallax, 768 milliarcseconds)
- NASA: Voyager 1 (about 17 km a second)
- NASA: Parker Solar Probe makes history with closest pass to the Sun (24 December 2024)
- Gaia Collaboration 2018, Observational Hertzsprung-Russell diagrams (Pleiades at 135.8 pc), Astronomy & Astrophysics 616, A10
- Menten et al. 2007, The distance to the Orion Nebula, Astronomy & Astrophysics 474, 515
- GRAVITY Collaboration 2019, A geometric distance measurement to the Galactic center black hole (8.18 kpc), Astronomy & Astrophysics 625, L10
- NASA: Messier 31 (the Andromeda Galaxy)
The Moon, Sun and Saturn distances are for November 7, 2026, computed with Astronomy Engine; this page refreshes every hour. For where things are tonight, see what’s in my sky tonight.