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Light Years

The distance between stars and galaxies in the universe is so vast it would be unwieldy to describe it in miles?like measuring the distance from New York to Tokyo in inches! Instead, scientists use light-years to measure distances in space. This sounds like a unit of time, but a light-year is actually a distance: the distance that light travels in one year.

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.


The speed of light
If you could drive nonstop to the sun at 60 mph, it would take 180 years. Light makes the same trip in eight minutes. So the sun is about eight light-minutes away.

The distance of a light year
How long would it take the space shuttle to go one light-year? The shuttle orbits the Earth at about 5 miles per second (18,000 mph). Light travels at 186,000 miles per second, which is about 37,200 times faster than the shuttle. So the shuttle would need about 37,200 years to go one light-year.

  The telescope as Time Machine
Did you know that when you gaze up at objects in the night sky, you?re looking back in time? How far back you see depends on how long it?s taken light from that object to reach you. The farther the object, the farther back in time you see.


Light from the moon?which is very, very close in space?takes about 1.3 seconds to travel to Earth. So you see the moon as it looked just over a second ago!


Credit: EIT - SOHO Consortium, ESA, NASA

The sun is much further; its rays take about eight minutes to reach Earth.



Saturn is an average of 10 times farther from the sun than Earth. We see Saturn as it was about 80 minutes ago.

Once you look beyond our solar system, objects are so far away it takes more than hours or even days for light to reach us. We?re seeing objects as they lookedyearsago. Click the images below to look through the telescope at objects in space?and find out how far back in time we?re looking.
Telescope ImageMap

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