# Sundials (was: Re: Time drift in amd64)

```On Fri, Mar 04, 2005 at 10:37:17AM -0500, Lennart Sorensen wrote:
> On Fri, Mar 04, 2005 at 10:24:30AM +0100, Martin Dickopp wrote:
> > Both of you are joking, I take it. But just in case someone is tempted
> > to take this suggestion serious: The sun position does *not* peak at
> > 12:00 "winter time" or 13:00 summer time. The deviation can be an hour
> > or more, and furthermore it changes every day.
>
> The sun does peak the same time every day, but it's only at noon exactly
> if you are in the right place on the planet (for your time zone).  If
> you are east or west of that the time will be off a bit, but it will
> still be the same every day.

Actually, that's not true. It varies quite a bit, as Martin
said. Take a look at the Equation of Time[1]. The variation isn't
anything like as much as the hour that Martin said -- it's about +/-
15 minutes over the year.  The reason it varies is (IIRC) to do with
the fact that the Earth's orbit isn't circular.

> The earth is pretty consistent in rotating
> at a steady speed.  Otherwise sundials wouldn't work very well.

Basic sundials such as those seen in many people's gardens *don't*
work very well, for the very reason given above (and the fact that
they're rarely set up properly). A good sundial installation will
always have some method of correcting for the current position in the
equation of time, usually either by having a date-driven graticule on
the plate, or by having a suitably-shaped gnomon. I've also seen
sundials with the equation of time inscribed in the plate, so that you
can do the correction manually.

Hugo.

[1] http://www.sundials.co.uk/equation.htm

--
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
PGP key: 1C335860 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
--- I always felt that as a C programmer, I ---
was becoming typecast.
```

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