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Re: Refraction
From: Clive Sutherland
Date: 2005 Aug 9, 00:09 +0100
From: Clive Sutherland
Date: 2005 Aug 9, 00:09 +0100
Marcel;
The original message you have quoted below contains
an error for which I apologise.
I should have said that the first
formula given as Ref =... is for use with Standard Temperatures and
Pressures, the values of which are quoted in the Nautical Almanac
as '10 deg C ' and '1010mbs.' These are clearly at sea
level.
Where any Abnormal atmospheric conditions are
suspected by the observer, Abn = ... should be used.
i.e. it is intended to correct for temperatures and pressures different
from the above.
The 'Abn' formula is quoted by GGBennet as correct
within 0.2 min between -20 < Air Temp <
+40, and 970 <air pressure mbs < 1050.
I have now retrieved the reference
above The full title is ''The Calculation of astronomical
Refraction in Marine Navigation. G.G.Bennet. J.Inst.Nav.(35,255).May
1982.'
However I do not know the complete source of the
correction formula, It may originate with Bennet. But I am of the opinion
that both formulae are applicable only when the observer is close to sea
level.
The following references may also be of use to you,
although they are not explicitly for negative observed altitudes
or for large observatory elevations.
1. 'THE COMPUTATION OF ANGULAR ATMOSPHERIC
REFRACTION AT LARGE ZENITH ANGLES'. by C.Y.Hohenkirk and
A.T.Sinclair. H.M.Nautical Almanac Office Technical Note No 63.Apr
1985.
2. 'ASTRONOMICAL REFRACTION'. by M.E.Thomas and
R.I.Joseph. John Hopkins APL Technical Digest. Vol 17 No 3.(1996). This is quite
good for Sunset observations as also is
3. REFRACTION NEAR THE HORIZON'. by B.E.Schaefer
and W Liller. Pub in 'The Astronomical Society of the
Pacific' 102. 796-805, July
1990.
If I can help more directly my email is clive@jcsutherland.wanadoo.co.uk
Regards.
Clive.
----- Original Message -----From: Marcel E. TschudinSent: Monday, August 08, 2005 6:28 PMSubject: Re: RefractionClive wroteThe following is one I use,Ref = Tan (90-Altitude - 0.999139 * Altitude - (7.31 / (Altitude + 4.4)))between -20 < Air Temp < +40, and 970 <air pressure mbs < 1050or else use the correction below for abnormal temperature and PressureAbn = (( Baro Pressure - 80 ) / 930) * (1 / ( 1 + 0.00008 * (Refraction + 39) * (Temperature - 10))),using mbs,deg celsius and deg Altitude.Refraction = Ref * Abn and True Altitude = Observed Altitude - RefractionDoes anyone of you know where this correction for abnormal condition comes from?What is actually ment with "abnormal"? As an example, the standard atmosphere also has temperatures below -20°C and air pressures below 970 mBar above a certain height, but this I would consider rather "normal". Could it be that the abnormal conditions refer to sea level conditions?Thanks Marcel
