NavList:

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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