Light-Years, Astronomical Units, and Parsecs: Converting Cosmic Distances
October 29, 2026
Distances in space are too large for kilometers to be convenient, so astronomers use bigger units. The astronomical unit (AU) is the average distance from Earth to the Sun, about 149.6 million kilometers (92.96 million miles). A light-year is the distance light travels in one year at 299,792.458 km/s: about 9.4607 trillion kilometers (5.879 trillion miles), or 63,240 AU. A parsec is about 3.26 light-years, or 30.86 trillion km, and is common in professional astronomy.
A light-year measures distance, not time. The nearest star system, Alpha Centauri, is about 4.37 light-years away, so light takes 4.37 years to reach us from it, and the distance is 4.37 × 9.4607 trillion = 41.3 trillion km.
Skip the math and get your answer instantly:
Open the Length Converter →Conversion Factors
1 light-year (ly) = 9.4607 × 10¹² km = 63,240 AU = 0.3066 parsecs. 1 parsec (pc) = 3.2616 ly = 206,265 AU = 3.0857 × 10¹³ km. 1 AU = 1.496 × 10⁸ km = 8.317 light-minutes. Light takes 8 minutes 19 seconds to travel from the Sun to Earth, 1.28 seconds from the Moon, and about 5.5 hours from the Sun to Pluto.
| Object | Distance (light-years) | Distance (km) | Light travel time |
|---|---|---|---|
| Moon | 0.00000004 | 384,400 | 1.28 seconds |
| Sun | 0.0000158 | 149.6 million | 8.3 minutes |
| Neptune (average) | 0.00047 | 4.5 billion | 4.2 hours |
| Alpha Centauri | 4.37 | 41.3 trillion | 4.37 years |
| Sirius | 8.6 | 81 trillion | 8.6 years |
| Center of the Milky Way | about 26,000 | 246 quadrillion | 26,000 years |
| Andromeda Galaxy | about 2.5 million | 23.7 × 10¹⁸ | 2.5 million years |
How Long Would It Take to Get There?
Travel time = distance ÷ speed. At 60 mph (96.56 km/h), the 41.3 trillion km to Alpha Centauri would take 41.3 × 10¹² ÷ 96.56 = 4.28 × 10¹¹ hours, or 48.8 million years. A jet at 900 km/h takes 5.2 million years. The fastest spacecraft, such as the Voyager 1 probe, travel at about 17 km/s (61,000 km/h), which would take about 75,000 years to reach Alpha Centauri. At 10% of the speed of light (30,000 km/s), a theoretical future probe would take about 44 years. These numbers show why interstellar travel is a challenge and why astronomers rely on telescopes.
Looking at distant objects is looking back in time: light from Andromeda left 2.5 million years ago.
Why Different Units?
Astronomers use AU for distances within the solar system, light-years in popular communication for stars and galaxies, and parsecs for professional work because parsecs relate directly to measured parallax angles: a star with a parallax of 1 arcsecond is 1 parsec away. For the largest scales, kiloparsecs (thousands), megaparsecs (millions), and gigaparsecs (billions) are used. The observable universe has a radius of about 46 billion light-years. The choice of units makes numbers manageable, like using miles instead of inches for travel.
Quick Mental Math
Light travels about 300,000 km/s, so about 1 foot per nanosecond. In a year there are about 31.56 million seconds, so a light-year is about 300,000 × 31.56 million = 9.46 trillion km. To convert light-years to miles, multiply by about 5.88 trillion. To estimate a travel time, divide the distance in km by the speed in km per hour, and then by 8,766 hours per year.
Frequently Asked Questions
How many kilometers are in a light-year?
About 9.46 trillion kilometers.
How many AU are in a light-year?
About 63,240 AU.
What is a parsec?
About 3.26 light-years, the distance at which a star shows a parallax of one arcsecond.
How far is Alpha Centauri?
About 4.37 light-years, or 41.3 trillion kilometers.
Is a light-year a unit of time?
No. It is a unit of distance.