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068

light-years

OLogy Series
astronomy
card
068

light-years

OLogy Series
astronomy

Would you measure the distance between New York and New Delhi in inches? For the same reason, astronomers don't use miles to measure vast distances in space. Instead, they use light-years: the distance that light travels in a year. That makes Proxima Centauri, the closest known star to the Sun, 4.28 light-years away; that's a lot easier than saying 24,000,000,000,000,000 miles.

Telescope or Time Machine?
Did you know that when you look up at a star or galaxy, you're looking back in time? The farther away the object, the longer it takes for its light to reach Earth. Although sunlight reflected off the Moon reaches us in seconds, light from the Sun takes about eight minutes to reach Earth. This means that you never see the Sun as it is right now, only the way it looked eight minutes ago. We see the nearest galaxy, Andromeda, the way it looked more than 2 million years ago. It takes billions of years for light from galaxies very far away to reach us -- this light started towards Earth when the Universe itself was still young. This is why we measure enormous distances in space by comparing them to how long it would take light to "go the distance."

What travels faster than light?

nothing

a jet plane

the Space Shuttle

Are you right?

Correct!

So far as we know, nothing can move faster than the speed of light. This is why we measure the largest distances by how long it takes light to make the trip.

Within our solar system, astronomers usually measure distances with:

light-years

Astronomical Units

meters

Are you right?

Correct!

An Astronomical Unit represents the average distance between Earth and the Sun -- 93 million miles. Using light-years to measure distances in our solar system is like measuring a grain of sand with a yardstick.

Mordecai-Mark MacLow, astrophysicist

The visible Universe is a big place. It is so big that even using light-years, one needs billions of them to measure its size.

A light-year is equal to 32 round trips between the Earth and the Sun.

Fact
or
Fiction
?

Fiction

Make that 32 thousand round trips. The Space Shuttle, which goes about 18,000 miles per hour, would cover one light-year in about 370 centuries.

What: distance light can travel in one year
Equal to: 5,900,000,000,000 miles
Significance: used by astronomers to measure large distances in the Universe outside of our solar system
Characteristics: a measure of distance, not time
First measured by: Ole Roemer

Image credits: Eric Hamilton; Mordecai-Mark MacLow: AMNH.