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Published: Sunday, July 14, 2013, 12:01 a.m.

AU's the simpler space measurement

When you consider just how far away the cosmos is the numbers can get so large that you can't process it. But you can make use of some handy units of measurements designed for astronomy.
Start with the mile. The diameter of the Earth is about 8,000 miles and the circumference of the Earth at the equator is just under 25,000 miles.
The closest celestial object, although human made, is the International Space Station that circles our world once every 90 minutes at a height of about 250 miles.
It resembles a super bright star generally pushing from the west to east at various heights. Keep up with the space station's comings and goings at www.heavensabove.com.
The next closest celestial body is the moon with an average distance of about 238,000 miles.
I can deal with hundreds of thousands of miles, but when it comes to millions of miles, the distance of the planets in our solar system, that's when I need help.
For example the brightest planet in the night sky right now is Venus, which is about 130 million miles away right now.
An easier way to express distances in our solar system is in astronomical units, and they're really simple. One astronomical unit (AU) equals 93 million miles, the average distance between the Earth and the sun. That would put Venus at about 1.4 AUs away.
Another planet in the early evening now is Saturn in the southwestern sky, a little above and to the left of Venus at 893 million miles away. In astronomical units that's 9.6 AUs away, which is a little easier to digest.
The next closest star to our sun is Proxima Centauri. That star is nearly 25 trillion miles away.
As humongous of a number as that is that's celestial chickenfeed compared to distances of other stars, many of which we can see every night with just our naked eyes.
That's why it's best to express stellar distance in light-years. A light-year is defined as the distance a beam of light travels in one year's time.
Using the speed of light, which is 186,300 miles per second, one light-year computes to 5.8 trillion miles. That would put Proxima Centauri at about 4.3 light-years away.
For example, the next brightest star closest to Saturn in southwest sky is Spica, just to the lower right of Saturn. Spica is 263 light-years away.
You wouldn't want to express that in miles, but there are some stars you can easily see that are thousands of light-years away.
In fact there are whole other galaxies of stars that more than 10 billion light-years away.
Light-years not only express distance but also time. Since a light-year is the distance light travels in one year, then the light that you see from a star that's 10 light-years away takes 10 years to reach your eyes.
The light you see from a star a 100 light-years away would take a century to reach you.
If a star is a 1000 light years away it would take a millennium for the light to reach you.
Mike Lynch is an astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of "Stars, a Month by Month Tour of the Constellations."
Story tags » Star Gazing

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