On Fri, Dec 19, 2008 at 3:00 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
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....
Exactly.
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).
True enough. This one is parallel to the ground and easy to make and set.
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.
Yes. That was the point I was driving at.
-ethan