“Light-year: Time or Distance?”

By Scott Hamilton

This week I wanted to write about a topic that came up in discussion with my kids and I realized that others might have the same question. I was asked how long a light-year lasts. It is a very common thing to think that a light-year measures an amount of time since it has “year” in the name, but sadly we are wrong with such an assumption. So now we need to learn where the term light-year comes from and why it is needed.

First the light-year comes from knowing the speed of light and how far light can travel in a given amount of time. Light travels at 186,282 miles per second, which is a number that can be pretty hard to comprehend, but using it as a measure of distance can make things much easier to understand when we start to measure things like the size of a galaxy, or the distance between two stars. The light-year is only one of the terms used for measurement of these astronomical distances. We can also use light-second, light-minute and light-hour, but light-year is the most commonly used and well known. Just so you know the numbers, a light-year is 5.88 trillion miles, a light second is 186,282 miles, a light-minute is 11.2 million miles and a light-hour is 670.6 million miles.

There is one other unit of measure used in astronomy that we can all understand a little better and that is the Astronomical Unit (AU), which is the distance from the Earth to the Sun, 93.2 million miles.

As he was researching ways to describe a light-year in terms that we could all understand, Robert Burnham noticed that there are 63,000 AU in one light-year and 63,360 inches in one-mile, so it made for a very easy method to bring light-years into a perspective that we could understand on earth. If we scale the AU to one-inch, we can then see the light-year as one mile. Now we can easily think about the distances in the universe on a level that we understand.

So let’s start with a few of the objects we see in the sky everyday. The Sun is one-inch away from the earth at this scale. The moon is then 1/350 inches away. As you can see the scale of the universe breaks our thinking rather quickly, especially since we see the Sun and Moon as both being fairly close to our planet, and sadly we are wrong, because we do not comprehend the size difference in the Sun and moon. The sun is 350 times farther away from the Earth than is the moon and the Sun is also about 350 times bigger than the moon, this is why total Solar eclipses can occur.

Now let’s take some of our other near neighbors in the Universe. Mars is 1.8 AU, so we can see it as 1.8-inches away. Pluto is 34 AU from earth, so it is just about 1-yard away at our scale.  Our closest star is Alpha Centauri, at 4.4 light-years away, which equates to 4.4 miles, followed by Sirus at 8.6 miles, and Vega at 25 miles. Next we can move on to some of the objects that are a little harder to see, but we know they are there. The center of our galaxy, the Milky Way, is 26,100 miles away and the three nearest galaxies in order are Andromeda – 2.54 million miles, Sombrero – 28 million miles and Whirlpool – 31 million miles. The farthest galaxy we know of today is 13 billion miles away, which begins to need another unit of measure to comprehend the distance.

If it is not clear yet, you can see how important a unit of measure the size of a light-year is to understanding the vastness of the Universe. So if you have grasped the idea of the light-year as it applies to distance, the next step is understanding what it means for our observance of time. Objects in the Universe being so far away from us means that we are not seeing them as they are, but we are seeing them as they were. When we look at the Sun we see it as it was about 8 minutes ago; when we look at Alpha Centauri we see it as it was 4.4 years ago; and when we look at the farthest known galaxy, we are looking into its past 13-billion years ago.

Next week I will write about what all this means to people who believe as I do in the young Earth and creationism theories, and how some of modern science is mistakenly beginning to prove these theories. Until next week stay safe and learn something new.

Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to sh*******@te**********.org or through his website at https://www.techshepherd.org.