# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Sun semidiameter**

**From:**Bill B

**Date:**2007 Apr 14, 23:08 -0400

Alex I see your point, that the N.A. lives up to it's accuracy claim. Past that it gets fuzzy for me. For the 23, 24, 25, August, 2007 pages N.A. SD is 15!8. For the 26, 27, 28 August, 2007 pages N.A. SD is 15!9. So it is safe to assume that mean SD will be 15!85 somewhere between the 25th and 28th. Yet we see SD's from other sources of: 23 Aug, 2007 15.82 24 Aug, 2007 15.83 25 Aug, 2007 15.83 26 Aug, 2007 15.83 27 Aug, 2007 15.84 28 Aug, 2007 15.84 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 18.9. --My $10 pocket calculator claims to display 10 digits but works to 11 digits and rounds the last up at 5. --My version of Excel claims to work to 13 figures and drops the last digit with no attempt to round up or down. --Even if, as you surmise, the N.A. may drop the last digit the result is still 15.8, not 15.9 based on Frank's numbers, and surveying numbers down to tenths of a second. I suspect something else is at work here, perhaps the N.A. algorithm? Bill Bill wrote > for 14 May 2007 I find agreement in the semidiameter of the sun; 15!83 > (Frank) and 15' 49.6" = 15!83 (Cadastral). Yet the nautical almanac lists > SD as 15.9. I see this happening on other dates as well. I would expect it > could happen near a rounding, point, say 15!9 one page and 16!0 the next, > but that thought does check out. > > Why the difference in SD? Alex Replied: The date May 14 is actually close to the date May 15/16 where the almanac data changes to 15.8 The explanation in the Almanac (p. 254) says that the almanac data are accurate to 0.1' which means that the difference between the tabulated value and the true value is less than 0.1'. In your example, asuming that the value given by Frank is exact, the difference is 0.07' which is consistent with the Almanac statement of accuracy. But this statement does not imply that the last digit is actually correct. So there is no contradiction. The reason of these errors is probably the rounding subroutine. To get the last digit really true, yoiu have to do all computations with one more digit, and then apply an exact rounding algorithm, while in most cases they just cut off the last digit instead. Because nobody really cares about the exactness of the decimal minute digit. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---