Value of One Light Year

What is a Light Year?

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Light - Year is the unit of distance denoted by a symbol ‘ly’. It is the distance traveled by a light in one year.


As defined by the International Astronomical Union, it is the distance traveled by a light photon in vacuum in one jubilan year.


In astrophysics, the term light - year term is often used to measure the distance between two astronomical objects. Such as the distance between the earth and the sun can be measured in terms of light - year.


The value of light - year in S.I. (metrics) is equivalent to :


9.4607 × 10¹⁵ m = 9.4607 Pm (1 Pm or petameter = 10¹⁵ m)


Light Year

Light - year or lightyear is the distance traveled by light in one Earth year.

A light year is a way of measuring distance just like inch, meter, kilometer or mile. That word ’year’ in ‘light year’ is a unit of time. 


However light - year is a measurement of distance.


Light Year is a Unit of

Light - year (ly)is a unit of length or distance. It is the length of distance a light covers in one year.


One Light Year Distance

One light - year distance is equal to 9.461e + 15 meters (m).


1 Light Year in Km

1 light year in km is equivalent to 9.4607 × 10¹² Km.


One Light Year in Miles

In Imperial and US units, one light - year is equivalent to 5.878 trillion miles or 5.878 × 10¹² miles which is equivalent to 9.5 trillion km.


1 Light Year in Days

As defined by the International Astronomical units (IAU).


Light - year is the distance traveled by light in a vacuum in one Julian year is equal to 365.25 days.


Therefore, one light - year = 365.25 days.


1 au in light years

1 astronomical units or au is equal to


1.5812507 × 10⁻⁵ or 1.5812e - 5 light - years


Distance From Earth to Sun in Light Years

Earth is the third planet from the Sun and is considered the only astronomical body known to harbor (support) life. 


In light years, the current distance of Earth from the Sun is:


0.00016062 or 1.6062 × 10⁻⁵ light years


How Many Kilometers is One Astronomical Unit au?

The distance of earth from the sun is about 149.6 million km.


1 astronomical unit or au is equivalent to 1.496e + 8 kilometers.


One Light - Year is Equal to How Many Years?

Basically, in a vacuum, the light travels at a speed of  1,079,252,849 km/h.

There are 8766 hours in a year.


Since Distance = Speed x Time = s x t…(1)


Therefore, to find the distance of a light - year, we need to multiply this speed with the number of hours in a year.


Given s = 1,079,252,849 km/h and t = 8766 hours


Putting value of s and t in eq(1) we get:


Distance = 1,079,252,849 x 8766


Hence, one light - year  =  5,878,625,370,000 miles.


What are Light Years Used for?

Measuring the astronomical distances, it becomes quite impossible and extremely cumbersome to measure these huge distances in miles or kilometers.

For example, in our cosmic neighborhood, the closest star-forming region to us is the  Orion Nebula which is at a short distance of 7,861,000,000,000,000 miles. We can rewrite this in light - years as 1,300 light - years away. 

Therefore, measuring the astronomical distances in light - years helps astronomers to determine how far back in time they are viewing.

Since light takes time to travel to our eyes which means that everything we view in the sky during nights have already happened. 

In other words, when we observe something one light - year away, we see it as it appeared exactly one year ago. 

As we can see the nearest spiral galaxy,  the Andromeda galaxy is 2.5 million light years away.

It means this galaxy appeared 2.5 million years ago.


Do You Know?

  • Light travels at a velocity of 3000 Km each second.

  • In a vacuum, it moves a speed of  670,616,629 mph or 1079,252,849 km/h.

  • Sometimes astronomers use a unit ‘Parsecs or pc’ as an alternative to light - year.

  • The distance between the galaxy and the earth is 65 million (mi) light - years or 382 quintillion (10¹² million) miles.

  • The nearest star to our sun is Proxima Centauri which is 4.2 light - years away.  

  • In one year, a light can cover a distance of about 10 trillion kilometers or 10⁶ Km.

  • In astronomical units (au), one light - year is equivalent to  63241 au or 0.3066 pc where au stands for astronomical unit and pc for Parsecs.

FAQ (Frequently Asked Questions)

Q1: Who Introduced Light - Year?

Ans: In 1838, the German astronomer Friedrich Wilhelm Bessel was the first to bring the concept of light - year.

He  used the light - year as a unit of measurement in astronomy to measure the astronomical distances. 

He measured the distance separating us from the binary star 61 Cygni that was equivalent to 10.3 light - years.

This distance came out to be 660,000 times the Earth's orbital radius.

Q2: Is Sun a Light Year Away?

Ans: The sun is 151.96 million kilometers away from the Earth.


Light travels 186,282 miles per second

A lightyear is the distance which is equal to speed x time.

= 186,282 miles per second x 60 seconds per minute x 60 minutes per hour x 24 hours per day x 365 days per year.

On solving we get one light - year = 5,874,589,152,000 miles. 

Therefore, the Sun is approximately 93,000,000 miles from Earth.

Q3: How Fast Does Light Travel?

Ans: 31 light - years is equal to 279 trillion kilometers or 18,60,000 AU. 

For this we need a vehicle (i.e., approximately not less than 1/1000th the speed of light to transport to this mammoth distance.

Therefore, it would take a minimum 31000 years to travel 31 light - years.