XPost: alt.usage.english   
   From: nospam@de-ster.demon.nl   
      
   Helmut Richter  wrote:   
      
   > On Thu, 19 Sep 2024, Christian Weisgerber wrote:   
   >   
   > > I'm sorry, I don't know where to post this.  I'm crossposting to   
   > > alt.usage.english, because statute miles as a unit mostly afflict   
   > > the English-speaking world.   
   >   
   > ... the English-speaking world with the exception of Ireland, Canada,   
   > Australia, New Zealand, and South Africa and some others.   
   >   
   > I think the term "United Kingdom and USA" would have been shorter and   
   > slightly more accurate.   
   >   
   > > So you want to convert between miles and kilometers.  The conversion   
   > > factor is... uh...  A 40-year-old calculator book provides a useful   
   > > tip:  Unless you're designing a space probe, you can use ln(5).   
   > >   
   > > WHAT?   
   > >   
   > > Yes, the natural logrithm of 5 approximates the conversion factor   
   > > between miles and kilometers; specifically one mile is about ln(5)   
   > > kilometers.  It's accurate to four digits.   
   > > If nothing else, it's faster to type on a calculator.   
   >   
   > And easier to remember than 1.609344. But when will you need such precision?   
   > For instance, when you drive on German roads outside villages, you must   
   > reduce your speed to 31.0685 mph whereas the rough rule 1 mi = 1.6 km would   
   > have allowed you 31.2500 mph.   
   >   
   > I like much more those thumb rules that actually allow you to estimate orders   
   > of magnitude, e.g.:   
   >   
   > 1 year ? π · 10? sec ? 31415926.54 sec   
   >   
   > or   
   >   
   > 1 year ? √10 · 10? sec ? 31622776.60 sec   
   >   
   > or, only for those wanting more precision, the arithmetic mean of the two   
   > which has three leading digits correct.   
      
   For orders of magitude there are 10^5 seconds to a day,   
   and 400 days to a year, so 40 million seconds to a year.   
   Both numbers are rounded up,   
   so the actual number is about 30 million seconds to a year.   
   All quite memorable.   
   The correct answer is 31 557 600 seconds/year (exactly)   
   so a quite acceptable estimate,   
      
   Jan   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)