# NavList:

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

**Re: Sun semidiameter**

**From:**Alexandre Eremenko

**Date:**2007 Apr 14, 19:55 -0400

Bill, 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. But I understand your concern:-) When you determine your IC from Sun, and then apply the "Soviet 0.4' test" you may be in trouble just because of the inaccuracy of the Almanac data. Indeed, the true 4SD computed from the Almanac can in principle differ from the true value by up to 0.4. (In your example it is 0.32' if we assume Frank's value to be correct to the last digit). Indeed, this makes the "0.4 test" sometimes somewhat too rigid. (I don't know to what precision they give this value in Soviet almanac, though:-) All this shows that in fact nobody (including the almanac publishers) is really concerned with higher accuracy than about 0.5'. (The days of the Lunars are gone, indeed!) And they hope that many various errors of the order of fractions of a minute will in most cases cancel each other. And they are right. I can give an example. Yesterday I measured a Lunar distance Sun-Moon GMT April 13 2007, 16:15:40, sextant reading 53d03.1, IC=-0.5. Then I reduce with Frank's calculator. The measurement turned out to be precise: Frank's calculator gives: error in the distance: 0.0', Error in longitude 0.7'. Then I reduce the same distance by hand, using the almanac (and Frank-computed altitudes). Using the algorithm outlined on the Frank web site. In the process of reduction I notice a lot of differences between Frank's numbers and my numbers (I use the almanac). Namely: Sun GHA: 63d46.5' (alm) 63d46.4' (Frank) Moon GHA: 112d27.3' (alm) 112d27.2' (Frank) Moon Decl: S12d37.6' (alm) S12d37.5' (Frank) True distance: 53d00.27' (my computation) 53d00.2' (Frank) Alt Moon corr'n: -49.4' (my) -49.6' (Frank). This is the largest discrepancy; I used the Moon tables in the almanac. The final result of my computation gives cleared distance 53d00.26' which is just 0.01' off the true value. That is 0.3' in longitude, while Frank says 0.7' in longitude. So you see all these plus-minus 0.1 errors usually cancel each other nicely in the process. And you can really use the almanac data for the Lunars and obtain very precise results. Not speaking of "practical navigation" for which the Complete on Board Celestial Navigator with its 1' rounding is probably adequate. Alex. On Sat, 14 Apr 2007, Bill wrote: > > > Looking at Frank's site > > http://www.clockwk.com/lunars/nadata_v5.html > > and Sun and Polaris Ephemerii�for Land Surveyors > > http://www.cadastral.com/2007ephs.htm > > 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? > > Bill > > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---