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Newsgroups: sci.lang,alt.usage.english
Subject: Re: OT: Converting miles/km
From: nospam@de-ster.demon.nl (J. J. Lodder)
Reply-To: jjlxa31@xs4all.nl (J. J. Lodder)
Date: Fri, 20 Sep 2024 22:24:26 +0200
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Helmut Richter <hr.usenet@email.de> 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