How big is the universe? How far away is the moon? How close is the next galaxy? The distance between objects in the universe is so vast that it is not practical to measure in miles or kilometers. Astronomers measure the distances in space by the distance light travels in a given amount of time. You may have heard the term "light year." Let's explore what a light year is.

Light can travel at a speed of 300,000 km (186,000 miles) a second. That means that in one second, light travels the distance you would cover if you traveled around Earth 7.5 times! The moon is about one light second away. Let's prove this.

Image shows the distance between Earth and the moon.

In order to calculate the distance between the Earth and the moon in light seconds, divide the distance between Earth and the moon by 300,000 km or 186,000 miles (the speed that light can travel in a second).

Check Your Answer 384,403 km ÷ 300,000 km = 1.281 or 1.3 light seconds

238,857 miles ÷ 186,000 miles = 1.284 or 1.3 light seconds Close Pop Up

What about objects that are farther away? How far do you think light travels in a minute? To figure this out, multiply the distance that light travels in a second by 60 because there are 60 seconds in a minute.

Check Your Answer 300,000 km * 60 seconds = 18,000,000 km/minute

186,000 miles * 60 seconds = 11,160,000 miles/minute Close Pop Up

So light can travel 18,000,000 kilometers in one minute!

Let's see how many light minutes Earth is from the Sun. In order to calculate the distance from the Earth to the Sun in light minutes, divide the total distance between the Earth and the Sun by 18,000,000 kilometers or 11,160,000 miles per minute.

Image shows the distance between Earth and the sun.

Check Your Answer 149,600,000 km ÷ 18,000,000 km/min = 8.329 or 8.3 light minutes

92,960,000 miles ÷ 11,160,000 km/min = 8.329 or 8.3 minutes Close Pop Up

What about light hours? How fast can light travel in an hour? To calculate how fast light can travel in an hour multiply the distance light can travel in a minute by 60 since there is 60 minutes in an hour.

18,000,000 km/min * 60 min/hr = 1,080,000,000 km/hour

So light travels 1,080,000,000 kilometers or 669,600,000 miles in one hour!

The distance from the sun to Saturn is measured in light hours. Let's see how many light hours apart the sun and Saturn are by dividing the distance from the sun to Saturn by 1,080,000,000 km/hour or 669,600,000 miles/hour.

Image shows the distance between the sun and Saturn.

Check Your Answer 1,433,900,000 km ÷ 1,080,000,000 km/hour = 1.327 or 1.3 light hours

891,000,000 miles ÷ 669,600,000 miles/hour = 1.330 or 1.3 light hours Close Pop Up

To calculate how far light can travel in a year, you need to multiply the light hours by 24 since there are 24 hours in a day and then by 365 since there are 365 days in a year.

Check Your Answer 1,080,000,000 km/hour * 24 hours * 365 days = 9,460,800,000,000 km/year

669,600,000 miles/hour * 24 hours * 365 days = 5,865,696,000,000 miles/year Close Pop Up

A light year is the distance light travels in one year. If we round our calculation above, we can see that a light year is about 9.46 trillion kilometers or 6 trillion miles!

Video segment. Assistance may be required. Watch the following video to learn more about light years.

Using light years to measure distances is advantageous to astronomers because if they used miles or kilometers, the distances would be so large that they would become impractical.

A light year can also be used to determine the age of a star or other space object. If a star is a million light years away and the light from that star travels at the speed of light, it would take the light a million years to get here. So, the star we are seeing is really how the star looked a million years ago, not how it looks today. The sun is 8.3 light minutes away. If the sun were to explode, we would not know about it for about eight minutes because that is how long it would take for the light of the sun to get to Earth.