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	FirstLight's First Ten Years
	29°39' N,  82°21' W Altitude: 50 Meters (more or less)		Updated Jan. 17, 1999
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Light Speed

by Howard L. Cohen

Elsewhere in this newsletter, George Russell, reflects on the "vastness of space." George notes that the light year is useful for simplifying measurements of long distances. He adds that he is also "quite comfortable with the light year." While I agree that a light year is a simple unit to use when giving large astronomical distances, I'm not sure I can easily comprehend vast stellar distances. Indeed, sci-fi episodes on TV or in the movies almost seem to shrink astronomical distances down to a short hop.

Not withstanding warp drives (whatever they are), consider light speed for a moment. During each second, radiation (e.g., light or radio) travels about 186,000 miles. That’s a bit fast for most earthlings to comprehend. Yet, this incredible speed shrinks in comparison to astronomical distances. The Moon is 240,000 miles from Earth (more or less). Simple division shows that radiation requires about 1.3 seconds to travel an Earth-Moon distance.

Make a call to your Moon buddies. Note that it takes nearly three seconds to answer your hello even if they respond immediately! Back home, have you talked to anyone overseas lately? Today, long distance telephone often goes by communication satellites about 22,000 miles up. Round trip time is about one second, long enough to confuse long distance conversations when both parties try to talk at nearly the same time!

Now consider old Sol 93 million miles away, a distance called an astronomical unit (AU). Divide the Earth-Sun distance by light speed and discover sunlight needs over eight minutes to reach the Earth. Pull the Sun’s "plug" and you won’t see the Sun go out for eight minutes! This latter number is the approximate light travel time for one AU. Remember it and you can quickly figure travel times to the planets at light speed. For example, when Voyager 2 passed through Saturn’s ring system, this giant planet was about 10 AU from the Earth. Earth-bound humans waited about 80 minutes before the radio signal arrived to say, "Hey, I'm OK."

Now mind-boggle yourself and realize that Pluto, usually the outermost planet, creeps around the edge of the solar system 40 AU from the Sun. Therefore, if you leave the Earth on a spaceship traveling at the speed of light, you will still need over five hours to reach the outer limits of our planetary system! Fortunately, as many FirstLight readers know (see the April issue), Pluto is presently near its closest point to the Sun, only 30 AU away! So, now is the time to get out your old light-speed space buggy for a journey to this small, cold world. Go now and you will shorten your trip by nearly 90 minutes!

For stay-at-home folks who have no speedy space buggies, remember this is the season to see Pluto, our outermost planet. Pluto reaches opposition on May 20. Shinning at magnitude +13.7, Pluto is bright enough to see in moderate size telescopes (8-in.?). Yes, dark, clear skies and some patience required. But give it the astronomer’s try in the next several weeks. Remember, we are the first humans in history to witness Pluto near its perihelion to the Sun. And don't forget, Pluto won't be this close or bright for nearly 250 years!

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