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On a recent trip to Central Australia I performed an observation of the sun
to determine local noon and thus longitude. This was done by performing a
series of observations about an hour before and an hour after what I thought
would be the time of meridian passage. On returning  to Sydney I was able to
locate my exact position on a map and was curious to see how close to local
noon my calculation was.

My definition of local noon is:

(1) When the centre of the sun crosses the observers meridian and

(2) When the centre of the sun is due north of the observer i.e. its Zn is
360d true and

(3) When the centre of the sun reaches its highest altitude

To test my results I went to the USNO site and plugged in the following
data:

Position        :       S 21d 48.3'     E 132d 40.0'

Date:            July 20, 2002

The USNO computer gave me the following results:
GMT             Hc              Zn
3h 23m 36s      47d 26.6'       357.3d
3h 15m 48s      47d 29.2'       359.9d
3h 15m 47s      47d 29.2        360.0d
3h 15m 40s      47d 29.2'       360.0d
3h 15m 38s      47d 29.2'           0.0d
3h 15m 35s      47d 29.2'           0.0d
3h 15m 34s      47d 29.2'           0.0d
3h 15m 29s      47d 29.2'           0.0d
3h 15m 28s      47d 29.2'           0.1d
3h 07m 42s      47d 26.6'           2.7d


Based on this analysis local noon was at 3h 15m 39s i.e. the mid point
between the equal altitudes of 47d 26.6'. I have several problems with this
answer.

Firstly how can the Azimuth of the sun move from 0d to 360d as it passes the
local meridian. I have checked my two compasses - a Francis  Barker
prismatic and a modern Silva baseplate and neither exhibits 0d. Isn't zero
degrees the absence of degrees? And aren't there 360d in a circle therefore
the sun bears 360d at local noon. By definition it doesn't move from 0d
to 360d. Further the USNO site shows the centre of the sun maintaining a
constant Zn of due north (0d or 360d) from 3h 15m 29s to 3h 15m 47s - a
period of 18s. While the sun's altitude does not change at culmination its
progress across the sky certainly does. I do not believe it is stationary
for 18s.

I may have made a mistake and would appreciate input (or possibly the sun
behaves differently down here). We have just lost the Rugby World Cup so the
sun may never shine in Australia again.

Kieran Kelly

   

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