Well, the first question is what kind of a year as there are several definitions, common, leap, Julian, Gregorian, anomalistic, sidereal, tropical, Besselian, etc. They differ by a few minutes but light moves pretty fast. (NIST uses 86400 seconds per day, and 1/100 of a Julian century, 36525 days). Does EVERYONE agree on which year is used as the time basis of a light year? (31 557 600 s)
Stellar distances are not measured with any great accuracy, being determined either by parallax or red shift. A light year works out to about 9.46 Pm (9.460 730 472 Pm, if you multiply out relationships above). Given accuracy limits, saying a lightyear is about 10 Pm (petameters) is not off by much. Knowing that the radius of earth's orbit is about 150 Gm, the scale between solar and interstellar distances is clearer to me than the mix of measurement units now used. ________________________________ From: Paul Rittman <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Sat, April 2, 2011 8:36:43 PM Subject: [USMA:50257] Astronomical measurements Now my next question isn’t really an objection to the metric system, so much as an interesting application of it. I can see how it would make sense to use metric-only measurements for earthly measurements. But I’m still of the opinion that “light-years” are the best measurement for astronomical distances. I know that the “parsec” is also used, but its only 3 times longer than the light-year, and not nearly so intuitive (I mean, to say “light-year” conveys the distance being measured). But I realize that the concept of light-year is not based on the meter. So would you folks advise (1) being purist, and quoting inter-stellar distances as “ultra-giga-multi-meters” or whatever; (2) in parsecs, which is almost entirely unknown to civilians; or (3) stick with the term “light-year”? And just as importantly, why?
