Lux, James P
james.p.lux at jpl.nasa.gov
Mon Oct 27 14:08:38 UTC 2008
On 10/27/08 3:37 AM, "Mike S" <mikes at flatsurface.com> wrote:
> At 01:01 PM 10/26/2008, Lux, James P wrote...
>>>> The world is round.
>> In the context of time-nuts, where we denigrate mere 1 ppm accuracy
>> and talk
>> about parts in 1E12 and more.. The Earth, being ellipsoidal by about a
>> in 300, is hardly "round".
> And this affects local solar time exactly - how?
My original comment was more along the lines of a humorous comment about the
use of the word "round" on a list where we deal in parts in 1E12..
BUT, since you ask.
The ellipsoidal nature raises all sorts of interesting questions when
calculating the apparent direction of the sun. Certainly, it will change
sunset and sunrise times, relative to a spherical earth. Noon is noon, so
the azimuth is always 180 or 0, but the elevation will change. However,
since in a time zone, there's only a single line where noon corresponds to
local solar noon, I suspect there is some small difference in timing between
the times when the sun appears highest in the sky for sphere and ellipsoid.
It's left as an exercise for the reader to determine whether the difference
in time is big enough to measure. (angular measurements of the sun's
position are readily possible to fractions of a minute of arc, with a
telescopic sight.. And it moves, to a zeroth order, about a degree in 4
For what it's worth, the solar noon/local noon varies +/- 15 minutes during
the year anyway, but that's more due to the non-roundness of the Earth's
orbit around the sun. (in the parts per hundred area.. It varies over time)
In fact, there's an interesting paper from IoN (Shaw, 2002) describing
determining one's position using an analemma and a clock, so I would assume
that if one knows the position, you could determine the time.
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