NavList:

If R is the refraction for standard conditions, (10ºC and 1010 hPa), Ha is the apparent altitude and h = Ha-R, then:

Bennett, G.G.: 1982, The calculation of astronomical refraction in marine navigation, en Journal of the Institute of Navigation, 35, 255.

R = 1/60 / TAN( Ha+7.31/(Ha+4.4) )

Meeus:

R = (58.294*TAN(90-Ha) - 0.0668*pow(TAN(90-Ha),3))/3600

R = (58.276*TAN(90-h) - 0.0824*pow(TAN(90-h),3))/3600

Saemunndsson:

R = (1.02/(TAN(h+10.3/(h+5.11))))/60.0;

Chauvenet Vol I, chapter IV gives a very good view of the general law of refraction.

…

Answering to Brad, yes the iterative process will be something like:

      for( dH = 0, iter = 0;  iter < 255;  iter++ ) {

            Hs = Hs + dH;

            Ha = AlturaAparente( Hs,  ie,  Dip );

            R = Refraccion( Ha, T, P );

            dH = Hc - ( Ha - R );

            if( fabs(dH) < 1E-8 ) break;

            }

It is more complicated if parallax, for Moon and Sun, it will be taken into account.

Regards.

---

Andrés Ruiz

Navigational Algorithms

https://sites.google.com/site/navigationalalgorithms/

De: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] En nombre de Antoine Couette
Enviado el: viernes, 12 de marzo de 2010 17:20
Para: NavList@fer3.com
Asunto: [NavList] Re: Star - Star Observations

Hello to all,

There is an EXCELLENT refraction formula which gives refraction as function of unrefracted topocentric altitude.

Its use would simply avoid the requirement for the "loop computation" you have explined.

This formula is the so-called Saemundsson's formula which can be found on the Internet. Its accuracy is well under 6 arc-seconds.

It is also quoted in M. Jean Meeus's most celebrated book "ASTRONOMICAL ALGORITMS" in its Chapter on atmospheric refraction.

Best Regards to you all

Antoine M. Couëtte