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When we look outward into space, we’re looking backwards in time. That’s because light moves, at the speed of light. It takes time for the light to reach us.
But it gets even stranger than that. Light can be absorbed, reflected, and re-emitted by gas and dust, giving us a second look.
Light - Echoes - Astronomers - Way - Universe
They’re called light echoes, and allow astronomers another way to understand the Universe around us.
We’re all familiar with the idea of an echo. Sounds travel through the air, and then reflect off a distant object and return. You hear the original sound, and then the reflected sound. And from that reflection, you can learn about the reflecting surface. Is it close or far? What’s it made out of?
Moves - Speed - Meters - Second - Light
That’s because sound moves at a speed of approximately 343 meters per second. Light, on the other hand, moves at a speed of almost 300,000 km/s. Too fast for your eyes to see the reflection, but out in space, where objects can be many light-years across, astronomers can see spheres of light moving through clouds of gas a dust, echoes of powerful flares and supernovae.
The best example of a light echo is radar, used to bounce radio signals off objects to map them out. A radar consists of a transmitter, to send the signals, and then a receiver, to capture them again.
Moves - Radio - Pulse - Objects - Figure
Since you know how fast light moves, you can detect your radio pulse bouncing off objects, and then use that figure out how far away everything is from you.
Here on Earth, they’re used by boats and airplanes to navigate around, as well as weather tracking.
Astronomers - Radar - Distances - Planets - Surfaces
But astronomers use radar to find the distances to planets and map out the surfaces of asteroids. For example, when the asteroid 3200 Phaethon made its closest approach to Earth in December 2017, the Arecibo radio observatory...
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