Q&A: Light Year Defined

Question: Whenever I read articles about astronomy I encounter this term “light year” Could you please explain what that means? — JBD, Glendale, AZ

Answer: The term light year is often used by astronomers to express distances in space. At first read it sounds like a measure of time, but it’s actually a measure of distance.

Space is so incredibly big that conventional measures, like the mile or kilometer, fall short of being useful. You just get so many zeros that the numbers become incomprehensible. For example, the distance to the nearest star, Alpha Centauri, is about 39,900,000,000,000 km, or 20,000,000,000,000 miles. All those zeros tend to impede comprehension.

One light year is the distance light will travel in one year. Light moves at the incredible speed of 186,000 miles/second. In one year, it will travel about six trillion miles! So we can use this huge unit of measurement to express the distance to Alpha Centauri as just 4.3 light years. Light from that star takes 4.3 years to reach Earth.

Not only is this more concise, it also provides some insight into the space-time interval between there and here, and then and now. When you look at Alpha Centauri you are seeing it as it was 4.3 years ago. It’s taken that long for the light you see to get from there to here. For more distant objects like Sirius (see above) the time interval is  even  greater. When you look into space, you literally look back in time.