Explainer: Understanding Light Years

'Tosin Adeoti
4 min readFeb 23, 2024

Yesterday, I made a post about the fastest-growing black hole just discovered by astronomers. The idea of it being ’12 billion light-years away’ caused some confusion. So, in this explainer, I will attempt to explain that concept in as layman terms as possible.

Have you ever gazed up at the night sky and wondered about the vast distances between where you are standing and what you can see in the sky? About the distance between two small ‘lights’ you see in the sky? How far away are they exactly? From the beginning of time, humans have always wondered about them. In fact, there are records of people sitting down and simply arguing about these details. There is a story of the Polish astronomer Nicolaus Copernicus standing outside every day pondering these questions and just watching the directions of the movement of these heavenly bodies. So, how far are they, really?

Enter the concept of a light year — a fundamental unit of measurement in astronomy that helps us grasp the immense scale of the universe. Just like you measure the bags of rice in your house in kilograms (kg) and the distance between your house and your office in kilometers (km) or minutes (mins), astronomers measure the distance between celestial objects in space in light years (ly).

Contrary to its name, a light year isn’t a measure of time but of distance. It represents the distance that light, traveling at a staggering speed of about 299,792 kilometers per second) in a vacuum, covers in one year.

To understand this better, let’s use an analogy. Imagine traveling from Lagos to Abuja in a car at a constant speed of 60 kilometers per hour. It would take you approximately 13 hours to cover the distance of about 800 kilometers between the two cities.

Now, let’s apply this analogy to the scale of a light year. Instead of driving a car, picture traveling at the speed of light — 299,792 kilometers per second. In just one second, you would not just have reached Abuja from Lagos, but you would have traveled the entire distance all the way around the Earth (what’s called the circumference of Earth). And you would have done it 7 times! If instead of traveling in just one second and stopping, you keep traveling for a full year, imagine the unimaginable distance you would have traversed.

Think about that.

What you are thinking about is called one light year.

Now, let me clarify something here: When I say ‘travelling for a full year’, I mean the Julian year (used during the times of the Romans), not the Gregorian year (we use today). But there isn’t much difference. The Gregorian Calendar defines an average year as 365.2422 days long, while the Julian Calendar defines an average year as 365.25 days long. It becomes significant over thousands and millions of years, but for this explainer, you don’t need to worry too much.

To put the concept of light years into perspective, let’s consider the following examples:

  • Proxima Centauri: The closest known star to Earth, located in the Alpha Centauri star system, is approximately 4.24 light years away. This means that whenever you look up at night and see any light from Proxima Centauri, it took it over four light years to reach us. You know how you are having network issues on Zoom and those on the other end are hearing what you are saying 10 seconds after you have said it. Yeah, it’s the same with this star. What it sent over 4 light years ago, you are just seeing it.
  • The Milky Way Galaxy: Our home galaxy, the Milky Way, spans an estimated diameter of about 100,000 light years. That means light traveling across the Milky Way takes tens of thousands of years to traverse from one end to the other.
  • Andromeda Galaxy: The nearest spiral galaxy to the Milky Way, Andromeda, is about 2.537 million light years away. This staggering distance underscores the vastness of our cosmic neighborhood.
  • J059–4351: The fastest-growing black hole in recorded history, that is swallowing up the mass equivalent of one Sun every day, is 12 billion light years away. So, even if it swallows everything in its way, it will take 12 billion light years to get here. You don’t need to worry. You would be long dead to witness it.

Why are light years important? They are crucial in astronomy for measuring distances between stars, galaxies, and other celestial objects. They allow astronomers to map the universe, study the evolution of galaxies, and explore the mysteries of deep space which may be useful not just for our continuous survival but to stimulate their brains in helping us discover theories that could lead to innovative technological advancements. Additionally, we are a very curious bunch of living things and many of us have not given up the quest of knowing how the universe came to be, the age of the universe, and how it expanded over time.

The study of light years of these bodies continues to help us peel those layers.