Value Of One Light Year

Light travels at 186,000 miles or 300,000 kilometres per second in an empty space. To give you a simple idea of how fast this is, light can travel around the Earth seven times in just one second! Astronomers use the speed of light to determine how far away objects are in space. The distance that light can travel in one year is measured in light-years (ly).

What is a Light Year?

  • A light-year is a unit of distance.
  • It is the distance a light photon travels in the vacuum in one Julian year
  • Denoted by ly
  • Value of one light-year is constant. 
  • The term light-year is often used in astrophysics to measure the distance between two astronomical objects.
  • Light travels at a velocity of about 300,000 kilometres (km) per second. 
  • It can travel about 10 trillion km. 
  • More precisely, one light-year is equal to 9,500,000,000,000 kilometres. 
    • Having said this, you might wonder why one needs such a big unit of distance.
    • In the universe, the measurement in kilometres is too small to be used. 
    • To better understand the statement, let us consider the following example:
      • The distance between the Milkyway galaxy and the nearest galaxy, the Andromeda Galaxy, is 21 quintillion km. 
      • This is equal to 21,000,000,000,000,000,000 km. 
    • Isn’t this number too large to interpret? 
    • For the same reason, in our solar system, we describe distances in terms of the Astronomical Unit (AU)

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec.

Light Year Conversion

The value of one light-year can be numerically expressed in kilometres as:

One light-year, ly = 9.4607 × 1012 km


  • 1 light-year in miles = 5.88 × 1012 miles
  • 1 light-year in the Astronomical unit = 63241 au.

How many Light Years Away is the Sun?

The Sun is at a distance of 149.6 million kilometres away from earth. Same distance, when expressed in terms of the light-year approximates 15.54795 × 10-6 light year.

Thus, the Sun is at a distance of 15.54795 × 10-6 light-years from Earth.

Age Of Celestial Body Using Light Year

  • Using a light-year as a distance measurement unit has another advantage
    • It helps you determine the age!
      • Let us consider a star that is 1 million light-years away. 
      • The light from the star has travelled at the speed of light to reach us.
      • Therefore, the star’s light took 1 million years to get to us, and the light we are seeing was illuminated a million years ago. 
      • So the star that we see is really how the star looked a million years ago, not how it looks today. 
    • Similarly, our sun is 8 light minutes away. 
    • If the sun were to suddenly explode right now, we wouldn’t know about it for eight minutes because that is how long it would take for the light of the explosion to get here.

Hope you have learned the value of one light year, along with the value of a light year in various other units.

Physics Related Topics:

Frequently Asked Questions – FAQs

What is the velocity at which the light travels?

Light travels at a velocity of about 300,000 kilometres (km) per second.

At what distance is the sun from the earth?

The Sun is 149.6 million kilometres away from the Earth.

What is the value of one light year when expressed in kilometres?

One light year = 9.4607 × 10^12 km.

Can the age of the celestial body be determined using a light year?


What is the value of one light year in Astronomical unit

One light-year is the Astronomical unit = 63241 au.

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