# How Long Is A Light Year

by on December 27, 2009

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This visible-light image shows the galaxy dubbed UGC 3789, which is 160 million light-years from Earth. Credit: STScI

Some people who ask the question, “How long is a light year?” actually think light year is a unit of time. Chances are, you found your way to this webpage with exactly the same idea in mind. A light year is actually a unit of distance. It is equivalent to the distance traveled by light in one year’s time.

We can calculate this distance by starting with the speed of light in vacuum. Space, after all, is mostly devoid of matter and hence, a vacuum. The speed of light in vacuum is 3×108 m/s. We simply need to multiply this value with the number of seconds there are in one year and we’ll get the equivalent distance expressed in meters.

We know that there are 60 seconds in a minute and 60 minutes in an hour. We also know that there are 24 hours in a day and roughly 365.25 days in a year. Therefore, 60 x 60 x 24 x 365.25 is equivalent to 31,557,600. In other words, there is roughly 31,557,600 seconds in one year.

Multiplying this value with the speed of light in vacuum, we obtain 9,467,280,000,000,000 m or 9,467,280,000,000 km. This is the answer to the question “How long is a light year?”.

Just how far is this? Well, if you run around the Earth at the equator, you’ll end up covering only 40,075 km. Or if you were to travel from the Earth’s core to the Moon’s core, the entire distance (including the distance separating the two celestial bodies) you will have covered is just 384,403 km.

Obviously, 1 light year is used to describe very large distances. For instance, you can use it to describe distances between stars. Alpha Centauri, for example, which is the nearest star system to ours, is 4.37 light years away.

The Milky Way, the galaxy to which our Solar System belongs, spans a diameter of roughly 100,000 light years.

Want really large values using light years? How about the Sload Great Wall – the largest known structure in the entire Universe. It is a giant wall of galaxies, also known as a galactic filament.

There are also other units of distance used in astronomical measurements. There’s the astronomical unit or AU and the parsec. However, due to pop culture, the light year has a more familiar ring.

We have some related articles here in Universe Today. Here are the links:

Here are the links of two more articles from NASA:

Here are two episodes at Astronomy Cast that you might want to check out as well:

Source: NASA

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