But how far exactly is a light-year? Light travels 186,000 miles per second. There are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 365 days in a year. Multiply these together to get 31,500,000 seconds in a year. Multiply that times 186,000 miles per second and you get 5,850,000,000,000 miles?about 6trillionmiles.
http://school.discoveryeducation.com/schooladventures/universe/itsawesome/lightyears/
View full size image. The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second.
http://www.space.com/15830-light-speed.html
One astronomical unit equals about 150 million kilometers (93 million miles). Another way of looking at it: the astronomical unit is a bit more than 8 light-minutes in distance. Quite by coincidence, the number of astronomical units in one light-year and the number of inches in one mile are virtually the same. For general reference, there are 63,000 astronomical units in one light-year, and 63,000 inches in one mile.
http://earthsky.org/astronomy-essentials/how-far-is-a-light-year
So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end. Astronomers measure the distance to the Andromeda galaxy using light-years. One light-year equals about 6 trillion miles. This galaxy is over 2 million light-years from Earth.
http://earthsky.org/space/what-is-a-light-year
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. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers).
http://science.howstuffworks.com/dictionary/astronomy-terms/question94.htm
A light year is the distance that light can travel in a year, or: 186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year. A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way! 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.
http://science.howstuffworks.com/dictionary/astronomy-terms/question94.htm
Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes.
http://www.space.com/15830-light-speed.html
So, in a year, light travels far. The Orion Nebula, 1,500 light years from Earth. Image credit: NASA/JPL-Caltech/STScI. Stars other than our sun are so far distant that astronomers refer to their distances not in terms of kilometers or miles – but in light-years. Light is the fastest-moving stuff in the universe. It travels at an incredible 300,000 kilometers (186,000 miles) per second. That’s very fast.
http://earthsky.org/astronomy-essentials/how-far-is-a-light-year
Hint: A light-year is a unit of distance, not time. A light-year is how astronomers measure distance in space. It’s defined by how far a beam of light travels in one year – a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong.
http://earthsky.org/space/what-is-a-light-year
He concluded that light took ten to eleven minutes to travel from the sun to Earth, an overestimate since it in fact takes 8 minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s).
http://www.space.com/15830-light-speed.html
how long per light year
A light year is at 9,460,800,000,000 kilometers. The light-year is 9,460,800,000,000 kilometers long.