© Norman Sperling, October 24, 2011
My friend John Westfall, an astronomer and geographer, points out that astronomy uses several non-metric units, most prominently the "light year". Officially, that's the distance that a beam of light travels, at the speed of light, in a year's time. In metric units, that's 9,460,730,472,580.8 km (about 9.5 Pm), according to Wikipedia.
While the light year is more than 5% shorter than 10^16 meters, no celestial object more than 20 light years away has its distance known within 5%. Uncertainties out there begin at 15% and quickly grow worse than 25%.
So, as far as anyone can measure, there is no difference between "100 light years" and "10^18 meters". Let's call 10^16 meters a "metric light year".
