Timekeeping (was Re: Sherwood Micro CPU/100)

Tony Duell ard at p850ug1.demon.co.uk
Fri Dec 19 14:00:30 CST 2008

> > While you don't get time as accurate as moden clocks
> > sundial and moondials are still interesting time telling devices.
> Yes they are.  I've only ever fiddled a little bit with telling time
> by the moon, but I did build a sundial at Pole - it was easy - the
> normal calculations all cancel out (think of "tangent of 90 degrees"),

Err, tan(90 degress) is infinite....

> and I effectively made what looks like a dartboard, divided into 24
> equal-angle slices.  Piece of cake (or is that pi?)

You can make such a dial at any latitude if you include the dial plate at 
the apporporate angle (the dial plate is parallel to the equator, hence 
the name 'equatorial dial). There's a fairly modern one in St Catherine's 
Dock in London (near Tower Bridge). 

At the pole, the equatorial dial and horixoantal dial are quite clearly 
the same thing. So the conventional idea of a sundial -- that is with the 
dial plate horizontal -- is an equatorial one at the pole.


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