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| | Astronomy Essentials on Aug 31, 2013

How far is a light-year?

Light is the fastest-moving stuff in the universe. It travels at an incredible 300,000 kilometers (186,000 miles) per second. So, in a year, light travels far.

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. If you could travel at the speed of light, you would be able to circle the Earth’s equator about 7.5 times in just one second!

A light-second is the distance light travels in one second, or 7.5 times the distance around Earth’s equator. A light-year is the distance light travels in one year. How far is that? Multiply the number of seconds in one year by the number of kilometers (or miles) that light travels in one second, and there you have it: one light-year. It’s about 9.5 trillion kilometers (5.88 trillion miles).

The Orion Nebula, 1,500 light years from Earth. Image credit: NASA/JPL-Caltech/STScI

Few of us can comprehend such a humongous number. Is there any way for us mere mortals to really understand how far a light-year is?

As a matter of fact, there is. The 20th century astronomer Robert Burnham Jr. – author of Burnham’s Celestial Handbook – devised an ingenious way to portray the distance of one light-year. He did this by relating the light-year to the astronomical unit – the Earth-sun distance.

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. This wonderful coincidence enables us to bring the light-year down to Earth. If we scale the astronomical unit – the Earth-sun distance – at one inch, then the light-year on this scale represents one mile.

Wikimedia Commons, Paul Stansifer, User:84

What is a light-year?

The closest star to Earth, other than the sun, is Alpha Centauri at some 4.4 light-years away. Scaling the Earth-sun distance at one inch places this star at 4.4 miles (7 kilometers) distant.

Scaling the astronomical unit at one inch, here are distances to various stars, star clusters and galaxies:

Alpha Centauri: 4 miles

Sirius: 9 miles

Vega: 25 miles

Fomalhaut: 25 miles

Arcturus: 37 miles

Antares: 600 miles

Pleiades open star cluster: 440 miles

Hercules globular star cluster (M13): 24,000 miles

Center of Milky Way galaxy: 27,000 miles

Great Andromeda galaxy (M31): 2,300,000 miles

Whirlpool galaxy (M51): 37,000,000 miles

Sombrero galaxy (M104): 65,000,000 miles