NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: formula for refraction
From: Frank Reed CT
Date: 2007 Mar 22, 18:41 -0700
From: Frank Reed CT
Date: 2007 Mar 22, 18:41 -0700
Bill, You changed one too many ones to twos when you edited the formula: toward the end, you've got "INT(B2)+2" where it should be "INT(B2)+1". That seems to be the only problem. You wrote: "Note: "INT" in my Excel rounds down the nearest integer." This you can count on. INT(x) goes back to the earliest programming languages. By contrast, VLOOKUP(x,a,i) is a relatively modern addition to spreadsheet functionality so you have to double-check the way it works. And you asked: "As a follow up, what is your opinion of the Saemundssen quoted in Meeus for Hc to H (below) as opposed to 57"/tanH" Well, I'm not quite sure what you're asking here. Those are not used for the same purpose. One thing to remember: equations like the one by Saemundssen that you mentioned are not "derived" from anything. They're just numerical "fits" to the tabulated data. Somebody just monkeyed with a simple equation and adjustable parameters until the equation's ouput looked like the table, more or less. If you have the table, you should *never* use those fitted equations. Likewise, if you have entered the table into a computer and implemented code or a spreadsheet function to interpolate, then you would always prefer that to an equation like Saemundssen. By contrast, the short equation for refraction above 15 degrees (r = 57"/tanH) is derived from fundamental physics given certain limiting conditions so we know *exactly* where it begins to be inaccurate (at some specified level of accuracy). -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---