NavList:

   

Reply

Alex wrote, about his observed errors with a box sextant-

| So where could these -3.5 possibly come from?
| This cannot be any anomalous dip (the whole dip correction
| was between -2.2 (when I sat on a stone just next to the water edge)
| and -3.5). It is hard to believe to such large arc irregularity at
| such moderate angles.

"Cannot" is rather overstating things, It's perfectly possible though
not very likely, for temperature gradients near the water surface to
give rise to such anomalous dip.  If, for example, warm air from over
the adjacent Sun-heated land blows over the surface of a cooler body
of water, that's a recipe for anomalous values for dip; significantly
worse than in open-sea conditions. Just because the dip itself is
small (because near the horizon) does not necessarily imply that such
refraction errors in the dip must also be small.

If Alex is certain that the weather was calm enough to dismiss Bill's
suggestion (overlapping waves on the distant horizon toward the head
of the fjord) then he is left with anomalous dip or a serious error in
marking (or reading) of the scale near the zero-degree point.  The
interesting thing would be to test again under very different local
weather conditions or in the open sea.

George

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

Reply