Light Years for Dummies (Metric System 2)

Distance to nearby stars

You readers out there who have taken science courses or higher mathematics (beyond simple arithmetic, in other words) have encountered the concept of "significant digits." You've probably been taught the basics of scientific notation and maybe even the difference between "accuracy" and "precision" (yes, there's a difference: you can look it up). Most staffers here at the Antisocial Network have math and science backgrounds, so they're aware that when calculating the circumference of a bicycle wheel it's not necessary to use ten digits of pi; a mere 3.14 will probably suffice. The freelancers who come from English Lit and Journalism backgrounds (the ones who opted to take neither math nor science after 10th grade) often have no idea of "significance" when throwing around very large or very small numbers. Take, for instance, eHow.com contributor Paul Ramone in his post "How to Convert Meters to Light Years."¹

Paul ("Joey's little brother?" we wondered) introduces the concept of a light year to his readers this way:

"A light-year is a unit of distance or length that is used most often to describe the distances between stars, planets, or other space objects. One light-year is equal to an extremely large sum when related to standard measurements used on Earth including the meter and kilometer."

Most people would be probably mention that a light year is the distance that light travels in a year, but not Paul. To him, it's "an extremely large sum"... well, not a sum, actually, but hey: word count! And just WTF is a "space object," anyway? Ramone continued to explain that "If you want to convert a figure in meters to light-years you can do so very easily with the help of a conversion calculator." Easy enough -- if you could run down to the store and buy a "conversion calculator," that is. Would a "regular calculator" work? Sure it would... OK, so on with the show: Here's how Paul says to convert meters to light years: "Determine the value of a light year and convert to meters." It's a safe bet that someone already (long ago) determined the "value" of a light year in meters. And hey! in the very next sentence Ramone tells us so!

"...one light year is equal to 9460528405000000 meters."

Hooo, doggies - that's a lot of meters! So when Paul tells us that to convert a distance in meters,

"...you simply need to divide the number of meters you have by the light year..."

He means (even though he doesn't actually say so) that

"...if you want to convert 500,000 meters to light years, [you] divide 500,000 by 9,460,528,405,000,000..."

Damn, Skippy! Our free solar-powered calculator from the paint store can't take all them there numbers! Haven't you ever heard of scientific notation? But Paul provides a reference to an online calculator, which tells us that 500 km (500,000 m) is 5.285004e-11 light years. Oddly, that is neither the answer you get with a calculator (5.28511e-11) nor the value that Ramone says is "the answer":

"In this case 500,000 meters is equal to 5.288624e-11 light years."

The difference between the calculated results and the website results arises because the website rounds the conversion factor to four significant figures – 9.461e+15 (three decimal places) – but Ramone rounds it to nine decimal places (9.460528405e+15). The difference between those answers and Paul's arises because he's a dumbass – a Dumbass of the Day. Of course, that website shouldn't be giving answers to seven significant digits, either...

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. The URL was   ehow.com/how_6160184_convert-meters-light-years.html

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MM - METRIC SYSTEM