NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sun semidiameter
From: Frank Reed CT
Date: 2007 Apr 14, 21:31 -0700
From: Frank Reed CT
Date: 2007 Apr 14, 21:31 -0700
Bill, you wrote: "If we take the average of the 23,24.25 page, 15.83 If we take the average of the 26, 27, 28 page, 15.84 No way either rounds up to 15.9" And so that looks like a difference of 0.07 minutes of arc --maybe something to worry about. But actually it results from a much smaller difference. After all, that 15.9 displayed value covers everything from just above 15.85 to just below 15.95. All those values round to 15.9. So if the result of the underlying calculation for the value in the N.A. was 15.853 (as an example), then the difference in the value from the other values you've found is only about 0.015 minutes of arc or one SECOND of arc. That's all you're seeing here. Because the Sun's semi-diameter changes so slowly, at the level of a tenth of a minute of arc, the displayed result of a calculation may differ by 0.1 minutes of arc for some days even though the underlying calculations are only one second of arc apart. Just doing some rough comparisons, this seems to be about right. It appears that the Sun SD in the Nautical Almanac is derived from a mean of 961 arc seconds while the value that I use (and Meeus, and lots and lots of other software and text sources) is 960 arc seconds. To get the Sun SD on any other date, you simply divide by the Sun's distance in AUs on that date. Of course, if you really want to drive yourself nuts with the variation of the Sun's semi-diameter, go to the table for the Sun's total altitude correction on the inside front cover of the Nautical Almanac. Suppose the Sun's observed altitude was 10d 00'. Suppose the date is March 31st locally, but April 1st in Greenwich. So what's the altitude correction? According to the table, interpreted very literally (!), the correction is 11.0 minutes of arc if it's March 31, but it's 10.7 minutes if the date is April 1. Take your pick. :-) -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---