At 11:39 AM 1/31/99 +0100, you wrote: >How about the dip angle which is there because your eye is not at ground >level and the calculations think you are! > >Dipangle(in minutes of arc)=1.06*SQRT(ht in feet) >Dipangle(in minutes of arc)=1.92*SQRT(ht in meters)
Hi Thibaud Where did you get these formulae from? I have used for ages another one dip['] = 1.76 * sqrt( a[m] ) or its older equivalent dip['] = 0.97 * sqrt( a[ft] ) where a is height of eye and units are given in bracket angles and ' stands for minute of arc rather than foot. The formulae include Earth curvature and mean terrestrial refraction for standard pressure 760 mm Hg (1013.2 mb or hP) air temp. +10 C water temp. +10 C (guess formulae may be used on land as well) The result should be corrected for temperatures and pressure if you are able to measure those, not that hard after all. "My" formulae are cited in manuals of navigation and Alamanachs together with tables and correction tables for temperatures and pressure. So where did you get "yours" from? Regards Slawek P.S. The so called terrestrial refraction refers to light rays arriving at low altitudes that is close to the horizon. Refraction for high altitudes is called astronomical. Physically this is the same phenomenon of bending of light rays passing through the atmosphere. Close to the horizon however air is "thicker" and influence of ground/sea and temperature differences is significant. In special conditions terrestrial refraction enables you to see really distant objects, sometimes inverted, and not only at Sahara. Both types of refraction have to be taken into account for sextant observations where celestial bodies are usually at high altitudes and apparent horizon is at low (and negative) altitudes. Slawek Grzechnik 32 57.4'N 117 08.8'W http://home.san.rr.com/slawek
