Ever heard someone say something like, “I feel like I’ve been waiting light years for this line to move?”
It’s a common mistake; one that is made daily by multitudes of people. A light year is not a measure of time but a measure of distance. That doesn’t make much sense because “light year” contains the word “year”, which is normally a unit of time. Even so, light years measure distance. It is the distance light travels unobstructed in a year, to be precise.
You know how long a foot or a meter is - you are comfortable with these units because you use them every day. Same thing with miles and kilometers - these are nice, human increments of distance.
When astronomers use their telescopes to look at stars, things are different. The distances are gigantic. For example, the closest star to Earth (besides our sun) is something like 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. That’s the closest star. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn’t a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!
So to measure really long distances, people use a unit called a light year. The speed of light is equal to 186,000 miles (300,000 kilometers) per second (also known as a light-second), and therefore a light year is 5,865,696,000,000 miles, or 9,460,800,000,000 kilometers (186,000 miles/second x 60 seconds/minute x 60 minutes/hour x 24 hours/day x 365 days/year = 5,865,696,000,000 miles/year).
The average distance from the sun to the Earth is referred to as an astronomical unit (AU), which is 92,955,807 miles (149,597,870,700 meters). However, to measure longer distances, astronomers use light years.
Using a light year as a distance measurement has another advantage; it helps you determine age. Let’s say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star’s light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, we are less than 8.5 light minutes from the sun. If the sun were to suddenly explode right now, we wouldn’t know about it for eight minutes because that is how long it would take for the light of the explosion to get here.
Neptune, our solar system’s current farthest planet, is 30.06 AU (or 2,794,251,558 est. avg. miles/ 4,504,000,000 km) from the sun. It only takes the sun’s light a little over 4 hours to reach Neptune. And on the outer edges of the solar system, the Oort Cloud, where comets are thought to originate, is 100,000 AU from the sun.