## How far is a lightyear?

By Marshall Brain

In the video, you have a chance to see how far a lightyear is. It really is an incredibly long distance -- a lightyear is roughly 6 trillion miles or nearly 10 trillion kilometers. Here's how you calculate it. You start with the speed of light, which is about 186,000 miles per second. Then you multiply that by 3,600 seconds in an hour. Then you multiply that by 24 hours in a day. Then you multiply that by 365 days in a year. And you get:
186,000 * 3,600 * 24 * 365 = about 6 trillion miles.
Scientists use lightyears to measure the really long distances between stars and galaxies. For example, a spiral galaxy close to our Milky Way galaxy is called the Andromeda galaxy. It is 2,200,000 lightyears away. One advantage of using the lightyear as a unit of distance is that you automatically know how long it took the light from the Andromeda galaxy to reach earth. We know that when we look at the Andromeda galaxy today, we are seeing light that was created 2,200,000 years ago! There are some galaxies that are billions of lightyears away.

Because 6 trillion miles is such a huge number, we really need to break it down to understand just how far it is. So let's start by looking at a light second. Since light moves at 186,000 miles per second, a lightsecond is 186,000 miles. The moon is about 1.3 lightseconds away. Since the circumference of the earth is about 25,000 miles, light could travel all the way around the earth 7 and a half times in one second.

A millisecond is a thouandth of a second. So a lightmillisecond is 186 miles. This is a distance we all can find easier to understand. In a thousandth of a second, light travels 186 miles.

A microsecond is a millionth of a second. So a light microsecond is about 0.186 miles, or roughly 1,000 feet.

A nanosecond is a billionth of a second. So a light nanosecond is about one foot, or 30 centimeters. Every nanosecond, light travels one foot. This is actually important in the computer industry, because lots of things inside modern computers can happen in a nanosecond or less. The maximum distance that light can travel in a nanosecond controls how big and fast computers can be.

What if we go the other direction? There is the lightminute. It is 60 lightseconds, or about 11 million miles. And a lighthour is 60 lightminutes, or about 670 million miles. Our solar system is about 8 lighthours across. In other words, it takes light roughly four hours to get from the sun to Neptune.

You can really impress your friends with this information. The next time someone says, "Hey, I just rode my bike 10 miles!", you can say, "that's about 53 lightmicroseconds. It would take light 53 microseconds to go 10 miles."

This table is a quick summary:

 lightnanosecond 1 foot 30 cm lightmicrosecond 1,000 foot 300 m lightmillisecond 186 miles 300 km lightsecond 186,000 miles 300,000 km lightminute 11 million miles 18 million km lighthour 670 million miles 1 billion km lightyear 6 trillion miles 10 trillion km

Have fun performing your own science experiments!