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Bill,
Thanks.
Pity you had no way to measure your height of eye.

Alex.

On Mon, 9 Apr 2012, Bill Morris wrote:

>
> Kermit and Alex
>
> Attached is a small spreadsheet of three sets of observations of dip made 
from Henderson Bay in New Zealand at 34deg 44.27minS , 173deg 06.6 min E. The 
azimuth 0 is a few degrees east of north. The horizon of azimuth 120 is 
before land about 25 km away. The approximate height of eye was 22 metres. I 
used a Wild T2 theodolite that reads to single seconds, but I do not think 
single observations are likely to be accurate to that precision. Seen through 
the 26 power telescope, the horizon is not a straight line but there are fine 
waves visible and on occasion the horizon appeared to move up and down a few 
arc seconds over a period of a few seconds. When this happened I checked the 
bubble for movement in case I had inadvertently disturbed the theodolite. I 
had not. In any case I checked the bubble for coincidence before and after 
making a reading. I think +/- 5 seconds would be a fair guestimate for the 
accuracy of the observations.
>
> On 8 April, the horizon appeared sharp to the naked eye and there were many 
fair-weather cumulus clouds in the sky. On the 9th the horizon was again 
sharp and the sky was cloudless. Air temperature on both occasions was about 
22 degrees. I did not measure the sea temperature. There was a light breeze, 
certainly not enough to move the theodolite, Alex.
>
> Plainly, the dip can vary from day to day and minute to minute, but with the 
limited data I can provide, I do not think one can comment about azimuth 
dependence. The observations do appear to underline that variable refraction 
sets a limit to the accuracy of altitude observations at sea.
>
> Bill Morris
> Pukenui
> New Zealand
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> Attached File: http://fer3.com/arc/img/118849.dip.xlsx
>
>
> : http://fer3.com/arc/m2.aspx?i=118849
>
>
>

   

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