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Great question! (Why values of 10 degrees Celsius and 1010 millibars)

The Nautical Almanac is silent on this topic and one of course then goes to 
the "Explanatory Supplement to the Astronomical Almanac and Nautical 
Ephemeris" volume of 1962 and its successor volume "Explanatory Supplement to 
the Astronomical Almanac" by Seidelmann of 1992.  The formula found in 
today's almanac is shown in the 1992 Seidelmann book, but not in the 1962 
version.  The only relevant comments therein are:

"The standard conditions used in The Nautical Almanac are T = 10 degrees C and 
P = 1010 mb; in The Star Almanac T = 7 degrees C and the pressure P = 1005 
mb" [Seidelmann, 1992, p. 145]

The formula shown in today's almanac is shown and referenced in Jean Meeus' 
Astronomical Algorithms in 1991 as being from G.G. Bennett's "The Calculation 
of Astronomical Refraction in Marine Navigation", an article found in the 
Journal of the Institute for Navigation, Vol. 35, 1982.

I do not have this article, so I am unable to further pin down why Bennett 
apparently chose these values of 10 degrees Celsius and 1010 millibars.

However, the choice of these is old, as they are the same used by the great 
W.M. Smart in his "Text-book in Spherical Astronomy" originally written in 
the 1920s.  He gives a good derivation of how to calculate refraction, with 
better versions found in both Seidelmann 1992 and Robin M. Green's 1985 
"Spherical astronomy", published by Cambridge.  Green, however, uses 0 
degrees Celsius and 760mm Hg as his "STP".  I particularly recommend Green 
for a readable, thorough treatment of many issues dear to this list.

Hope this helps, but I have not answered definitively your question!

Dan

-----Original Message-----
From  Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Chuck Taylor
Sent: Thursday, November 09, 2000 6:58 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: T & B Corrections


The T & B (Temperature & Barometric Pressure) corrections in the Nautical
Almanac are provided for occasions when temperature and barometric pressure
differ from the "standard" temperature of 10 degrees Celsius (50 degrees
Fahrenheit) and 1010 millibars (29.83 inches of mercury).  They are applicable
mostly to low-altitude sights and allow for the fact that refraction can vary
with T & B.

An aeronautical engineer friend of mine tells me than "Standard Temperature and
Pressure" in his world are 15 degrees Celsius (59 degrees Fahrenheit) and 1013
millibars (29.92 inches of mercury).

Does anyone know the logic for selecting 10 degrees Celsius and 1010 millibars
as the basis for the refraction tables?  There must be some historical reason,
but I can't find it in Bowditch.

Chuck Taylor
Everett, WA, USA

   

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