One light year is about 9.46 lakh crore kilometres. Here is how far that really is
A light year is a distance, not a time. We worked out how long it would take to travel one by train, by plane and by spacecraft, and how far India’s own missions are in light-time.
One light year is the distance light travels in one year: 9,460,730,472,580.8 kilometres, or about 9.46 trillion km. In Indian numbering that is roughly 9.46 lakh crore kilometres. Despite the name, a light year measures distance, not time. When a star is “100 light years away”, its light has been travelling for 100 years to reach you.
Numbers that large stop meaning anything, so we turned the light year into journeys. At the top speed of a Vande Bharat train it would take 6.75 million years. Voyager 1, the farthest spacecraft, would need about 17,700 years. Light does it in exactly one.
How is a light year calculated?
Two numbers multiply together. The speed of light in a vacuum is exactly 299,792.458 km per second; since 1983 it has been fixed by definition, and the metre is defined from it. For the year, the International Astronomical Union recommends the Julian year of exactly 365.25 days, which is 31,557,600 seconds. Multiply the two and you get 9,460,730,472,580.8 km, with no rounding needed.
The quarter day matters more than you might think. Light covers about 6.5 billion km in a quarter of a day, so a “light year” built on a 365-day calendar year would come out roughly 6.5 billion km short, which is more than the distance to Neptune.
How long would it take to travel one light year?
We divided the light year by the speeds of things people know. The results span nine powers of ten, so the chart below uses a scale on which each gridline is ten times the last.
- Walking at 5 km/h: about 216 million years. Had you set off in the Late Triassic, when dinosaurs were still a young group, you would be arriving about now.
- A Vande Bharat train at its 160 km/h top service speed: 6.75 million years.
- A passenger jet at 900 km/h: about 1.2 million years.
- The International Space Station, at 7.66 km per second: about 39,000 years.
- Voyager 1, at about 17 km per second: about 17,700 years. To reach Proxima Centauri at that speed would take some 75,000 years, and Voyager is not heading that way.
- NASA’s Parker Solar Probe at its record 192 km per second, the fastest any human-made object has travelled: about 1,560 years. And it only reaches that speed for a short time close to the Sun.
India in light-time
Light is so fast that the whole country is a blink. From Kanyakumari to the northern tip of Kashmir, India spans about 3,214 km north to south; light crosses that in 10.7 thousandths of a second. The straight line from Delhi to Mumbai, about 1,150 km, takes 3.8 thousandths.
- Chandrayaan-3 on the Moon: a command sent from Bengaluru took about 1.3 seconds to arrive.
- Aditya-L1, at the Sun–Earth L1 point about 1.5 million km away: roughly 5 seconds each way.
- Mangalyaan at Mars: between about 3 and 22 minutes each way, depending on where Mars was in its orbit relative to Earth.
On those scales the next step out is enormous. Voyager 1, launched in 1977, is only now reaching one light-day from Earth. One light year is 365 times farther, and the nearest star more than four times farther again.
Why astronomers use light years
The unit carries a built-in reminder: looking out is looking back. You see Proxima Centauri as it was 4.25 years ago, and the Andromeda Galaxy as it was about 2.5 million years ago, when the earliest members of our genus were appearing in Africa. Professionals usually prefer the parsec, 3.26 light years, because it falls straight out of the way stellar distances are measured, by the tiny shift in a star’s position as Earth goes around the Sun. Across the observable universe even light years get awkward, because space itself has stretched while the light was travelling; our story on how old the universe is explains why the most distant galaxies are now far more light years away than their light’s age.
Three common mix-ups
- “Light years ago” is a mistake: the unit is a distance. “Years ago” or “light years away” is what is meant.
- A light year is not 365 days of light travel. It uses 365.25 days, which adds about 6.5 billion km.
- A light year is not the farthest light can go. Light keeps going. The light year is simply a convenient ruler.
How we worked this out
The light year is the product of the defined speed of light and the IAU’s Julian year. Travel times divide that distance by each speed: 5 km/h walking, 160 km/h for Vande Bharat’s maximum operating speed, 900 km/h for a typical jet cruise, 7.66 km/s for the ISS at 420 km altitude, about 16.9 km/s for Voyager 1 relative to the Sun, and 191.7 km/s for Parker Solar Probe’s record. Light-times for missions use average or typical distances: 384,400 km for the Moon, 1.5 million km for L1, and the closest and farthest Earth–Mars distances of about 55 million and 401 million km. India’s north–south extent is the government figure of 3,214 km. The distance to Proxima Centauri is from the Gaia space telescope’s parallax measurement.
Sources
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