What is a light year?

When we look at the night sky, we can see celestial bodies, such as planets and stars, that are very far away from us! Measuring the distance between us and these celestial bodies would be expensive if we used the meter (m) and/or kilometer (Km) distance units, as the numbers would be too large. Therefore, the unit of distance used in Astronomy is the Light-year.

Generally, due to the presence of the word year, this quantity is confused as a unit of time, but light-year is a unit of distance and can be defined as the distance traveled by light in a year's time.

To measure the value of 1 light-year in kilometers, we start from the defined value of the speed of light, which is 300,000 km/s or 300,000,000 m/s. Knowing that velocity is the result of the reason of space for time, and making the transformation of 1 year in seconds, we have:

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1 year = 365 days.24h.3600s = 31,536,000s

V =  d
t

300,000,000 = d / 31,536,000 → d = 300,000,000. 31.536.000

d ≈ 9,460,080,000,000,000 m ≈ 9,461 .10¹⁵ m ≈ 9,461 .10¹² km

We can conclude that 1 light year is approximately 10 trillion kilometers!

The table below shows the distance between our planet and some places in the universe in two units:

* Alpha Centaur, after the Sun, is the closest star to Earth.

To get a sense of the immensity of the universe, realize that, traveling at the speed of light, to reach the center of our galaxy it would take approximately 23 million years!

By Joab Silas
Graduated in Physics

Would you like to reference this text in a school or academic work? Look:

JUNIOR, Joab Silas da Silva. "What is a light year?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/fisica/o-que-e-ano-luz.htm. Accessed on June 27, 2021.