How Earth years relate to light travel
A light-year is the distance light travels in one Earth year, so the two share the same number. If light travels for a number of years, the distance it covers, in light-years, is exactly that many years. This is why the distance to a star in light-years also tells you how many years its light has been traveling to reach us.
distance (light-years) = Earth years of travel
1 light-year = 9.4607 × 10¹² km = 63,241 AU
Worked example
Light from Proxima Centauri travels for about 4.24 years to reach us, so the star sits 4.24 light-years away, roughly 40.1 trillion km. The light you see tonight left the star more than four years ago.
Frequently asked questions
How far does light travel in one Earth year?
One light-year, about 9.46 trillion kilometers (5.88 trillion miles).
How many light-years does light travel in 10 years?
Exactly 10 light-years, because a light-year is defined as the distance light covers in one year.
Is a light-year a measure of time or distance?
Distance. It is how far light travels in one year, roughly 9.46 trillion km.
How long does light from the nearest star take to reach Earth?
Proxima Centauri is about 4.24 light-years away, so its light takes about 4.24 years to reach us.
How far away is a star that is 100 light-years distant?
About 946 trillion km (588 trillion miles), and its light left the star 100 years ago.
How do I convert Earth years to a distance in light-years?
Use the same number: light covers 1 light-year per Earth year, so 25 Earth years equals 25 light-years.