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Light Years Ahead

Bad Astronomy: "We're light years more advanced than our competition!"

Good astronomy: There is no good astronomy equivalent here. The phrase is just wrong!


How it works:
One of the reasons I loved astronomy as a kid was the big numbers involved. Even the closest astronomical object, the Moon, is 400,000 kilometers away! I used to convert that number into all sorts of units, like millimeters and Angstroms and inches. I was a geek, sure, but it was fun!

Unfortunately, in astronomy the numbers get big pretty fast. The Sun is 150,000,000 kilometers from the Earth. Pluto is nearly 5,000,000,000 kilometers away. The nearest star to the Sun is over 40,000,000,000,000 kilometers away. Think of what that is in centimeters!

There are a number of ways around using these awkward figures. One is to make your units bigger. Compare these two statements:

  • I am 17,800,000,000 Angstroms tall.
  • I am 1.78 meters tall.
  • Using meters is a much more realistic unit (an Angstrom is an incredibly small unit; 100 million of them would fit across a single centimeter!).

    So astronomers need a really big unit to measure distances to stars. Well, the speed of light is terrifically fast; a beam of light moves about 300,000 kilometers every second! It can travel to the Moon in just over a second, to the Sun in eight minutes, and even to distant Pluto in about 6 hours or so. It takes about 4.3 years to get to the nearest known star besides the Sun. A beam of light will travel 9,500,000,000,000 kilometers in one year. This distance is defined as a light year ; the distance light travels in a year.

    So the light year is a measure of distance, not time, even though it has the word "year" in it. This hasn't stopped some people from misusing it, however. I can picture some advertising executive yelling at his idea people some time ago, telling them that saying their product is "years more advanced than the competition" just doesn't cut it. One of the people sitting there timidly raises his hand and says, "How about if we say `light years' instead?", and some Bad Astronomy was born.

    (Note added July 28, 1999: Longtime Bad Reader Russ Bogel notes that ZoomTown internet has an ad that states ``ZoomTown is lightyears faster than a regular connection!'' Now advertisers use it as a speed! Wonders never will cease.)

    An addendum to this: In the movie Star Wars, Han Solo brags that he can make the Kessel run in less than 12 parsecs. This is another example of Bad Astronomy; like a light year, a parsec is a unit of distance, not time (to be specific, it equals 3.26 light years). To be fair, Obi Wan Kenobi gets a pained expression when Solo says it; maybe Kenobi knows a bit more astronomy than Solo does.

    Note added May 25, 2001: Okay, before you email me saying that the Kessel Run was a race through a warp in space, or through a black hole field, or whatever, PLEASE READ THIS PAGE! I think over the past two years I have received a thousand emails about this.

    Addendum II: I have received more email about this phrase than anything else in my Bad Astro pages. Most of these emails are the flavor of "You can actually use the phrase 'light years' as a distance and still say 'light years ahead', just like you can say 'miles ahead'." Of course, that's true. I still hold, however, that many people use the phrase to mean a unit of time. And if they are getting it correct, great! I still like having the chance to explain something about astronomy to folks! ;-)



    ©2008 Phil Plait. All Rights Reserved.

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