What is a light year in simple terms!

Light years are a measure of distance, not time (as the name suggests). A light-year is the distance traveled by a light beam in an earth year, which is 6 trillion miles (9.7 trillion kilometers).


On the scale of the universe, measuring distance in miles or kilometers does not solve the problem. Just like you can measure the distance to the grocery store by the time it takes to drive there (the grocery store is a 15-minute drive), astronomers use the time required for light to travel to measure the distance to the stars to us. For example, the nearest star to our sun, Proxima Centauri, is 4.2 light years away.

How far is a light-year?

Unlike the speed of a car when running errands, the speed of light is constant throughout the universe and is known for its high precision. In a vacuum, light travels at 670,616,629 miles per hour (1,079,252,849 km/h). To find the distance of one light-year, you can multiply this speed by the number of hours in a year (8,766). Result: One light year is equal to 5,878,625,370,000 miles (9.5 trillion kilometers). At first glance, this seems to be an extreme distance, but the sheer size of the universe dwarfs this length.

Why use light years?

It would be very cumbersome and impractical to measure in miles or kilometers on an astronomical scale. Starting from our cosmic neighborhood, the Orion Nebula, the nearest star-forming region, is only 7,861,000,000,000,000,000 miles away from us, or more simply, 1,300 light-years away. The center of our galaxy is about 27,000 light years away. The closest spiral galaxy to us, the Andromeda Galaxy, is 2.5 million light-years away. Some of the most distant galaxies we can see are billions of light years away.

Measuring in light-years also allows astronomers to determine how far they have been observing. Because light takes time to reach our eyes, everything we see in the night sky has already happened. In other words, when you observe something 1 light-year away, you will see how it appeared a year ago.

We saw the Andromeda Galaxy that appeared 2.5 million years ago. The most distant object we can see, the cosmic microwave background, is also our oldest view of the universe, after the Big Bang about 13.8 billion years ago.

Alternatives to light years

Astronomers also use parsec as a substitute for light years. Abbreviation for parallax second, which comes from the use of triangulation to determine the distance of the star. More specifically, it is the distance of the star whose apparent position in the sky is shifted by 1 arc second (1/3,600 degrees) after the earth orbits the sun halfway. One arc second is equal to 3.26 light years.

Like degrees, light years can also be broken down into smaller units, such as light hours, light minutes, or light seconds. For example, the sun is more than 8 light minutes away from the earth, while the moon is just over 1 light minute away from the earth. Scientists use these terms when talking about communications with deep space satellites or rovers. Due to the limited speed of light, it may take more than 20 minutes to send a signal to the Curiosity rover on Mars.

Whether it is light-years or parsecs, astronomers will continue to use both to measure the distance in our vast and vast universe.


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