Next message: Stacy Hanna: "Re: leap seconds a navigational hazard, says expert"
On Sat, 9 Aug 2003 22:52:32 -0700, Brooke Clarke wrote:
>
>I think you can get 15 seconds accuracy at any time the Sun is shining.
>There are a number of heliochronometers that do it. But it's not a
>static sundial, that's to say you need to turn dials, etc.
>
I like that one with the gnomon made of two plates curved and bent ...
almost meeting, but with a gap such that a light bar automatically
intersects with the equatorial ring to solve the equation of time.
One of the links on your site which I will have to visit again.
>In the
>extreme case you could use a surveyors transit/theodolite to shoot the
>az and el of the Sun and using data for the current year know exactly
>the time to about a second. See:
>http://www.cadastral.com/
>
From that site, a while ago, I downloaded "The Revenge of the Altitude
Sight". I believe the point is made that the time and longitude need
not be known exactly to determine the Azimuth of a point by using the
Sun if you take both Az and V readings each shot. This was common
with the old open circle vernier transits that were being made
obsolescent in the 1950s.
Using the now standard hour angle sight requires the Az of the Sun and
exact time, as well as a good guess for a latitude and longitude; but
can be used around local noon where the altitude sight could not.
Accuracy is increased because of less concern for refraction, and also
the optical theodolites read to the nearest second of arc. And, the
altitude need not be made tangent to the sun's upper or lower limb at
the same time as the Azimuth is tangent to the trailing limb.
OK, you can work backwards with both methods, but only the hour angle
one will work around noon ... if you set up accurately on a line of
known azimuth and have an ephemeris handy for that year.
I think a sundial is more practical, looks better, and can be used by
any person close enough to see the shadow or light spot. I like your
site and links ... maybe because they seem to agree with me? :-)
Many thanks,
--
Richard ...
