A light-year

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. If you like to keep up with what’s going on in astronomy, it’s worth spending a little bit of time understanding what the deal is with this funny unit of measurement.

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).

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.

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