Using a light year as a distance measurement has another advantage -- it helps you determine age. Photo Youtube

A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way!

Using a light year as a distance measurement has another advantage -- it helps you determine age. Let's say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so 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.

## What is a light year?

Light-year, in astronomy, the distance traveled by light moving in a vacuum in the course of one year, at its accepted velocity of 299,792,458 metres per second (186,282 miles per second). A light-year equals about 9.46073 × 1012 km (5.87863 × 1012 miles), or 63,241 astronomical units. About 3.262 light-years equal one parsec.

For most space objects, we use light-years to describe their distance. A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km). That is a 6 with 12 zeros behind it!

## Looking Back in Time

When we use powerful telescopes to look at distant objects in space, we are actually looking back in time. How can this be?

Light travels at a speed of 186,000 miles (or 300,000 km) per second. This seems really fast, but objects in space are so far away that it takes a lot of time for their light to reach us. The farther an object is, the farther in the past we see it.

Our Sun is the closest star to us. It is about 93 million miles away. So, the Sun's light takes about 8.3 minutes to reach us. This means that we always see the Sun as it was about 8.3 minutes ago.

The next closest star to us is about 4.3 light-years away. So, when we see this star today, we’re actually seeing it as it was 4.3 years ago. All of the other stars we can see with our eyes are farther, some even thousands of light-years away.

In 2016, NASA's Hubble Space Telescope looked at the farthest galaxy ever seen, called GN-z11. It is 13.4 billion light-years away, so today we can see it as it was 13.4 billion years ago. That is only 400 million years after the big bang. It is one of the first galaxies ever formed in the universe.

Learning about the very first galaxies that formed after the big bang, like this one, helps us understand what the early universe was like.

## Light Year Conversion

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

One light-year, ly = 9.4607 × 1012 km

Similarly,

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.

 Cool Fact A light nanosecond -- the distance light can travel in a billionth of a second -- is about 1 foot (about 30 cm). Radar uses this fact to measure how far away something like an airplane is. A radar antenna sends out a short radio pulse and then waits for it to echo off an airplane or other target. While it's waiting, it counts the number of nanoseconds that pass. Radio waves travel at the speed of light, so the number of nanoseconds divided by 2 tells the radar unit how far away the object is!­

## 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 that 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. In the same way, our sun is 8 or so 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.

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