A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Fred Hebard
Date: 2003 Mar 18, 14:56 -0500
George Huxtable wrote: > Thanks to Fred for providing data of his land-based observations, > which any > sextant observer would be proud of. It would be interesting to compare > those figures with what Fred could get out of shore-based > observations up > from a horizon, or those taken in real-life from a small boat. > > > It's notable that much of the scatter in Fred's positions occurs in his > altitudes of the Moon. Is Fred taking all the minor corrections for > the > Moon into account, such as augmentation of semidiameter, reduction of > parallax, I wonder? Normally, one would only bother with such matters > when > seeking the ultimate precision of a lunar, but Fred is achieving such > remarkably consistent answers that it may be worthwhile taking these > minor > matters into account. First, thank you George for giving me a reason to go sailing! That may be related to the ultimate purpose of my exercise... The moon is definitely more "variable" than the other bodies in my observations. Here is a summary (I know it's incorrect to average the standard deviations, but it's accurate enough with these data to demonstrate that the moon is more variable): ~~~~~~~~~~~~~~~~~~~~~Ho-Hc Object~~~N~~~mean~of~~~~~~mean~of ~~~~~~~~~~~~~block~~~~~~~~block~std ~~~~~~~~~~~~~means~~~~~~~~deviations Jupiter~~3~~~(-)0.53~~~~~~0.35 Moon~~~~10~~~(-)0.10~~~~~~0.66 Sirius~~~5~~~(-)0.63~~~~~~0.32 Sun~~~~~~6~~~(-)0.25~~~~~~0.30 Regarding moon errors, I am just using the standard Nautical Almanac corrections, and keying data in from the Almanac, rather than formulas, such as George outlined for Young's method of clearing lunar distances. The Almanac's corrections include reduction in HP with altitude; I can't remember offhand whether it still includes augmentation. I note that the Almanac's corrections vary from those of the U.S. Naval Observatory site. I'm not sure why the moon is so untrustworthy for me, although when it's good it's very good for use in a liquid artificial horizon. It doesn't heat the liquid and the wind tends to be calmer at night, allowing removal of the glass cover. It's also bright enough to see easily. In the larger data set, when the standard deviation of the moon shots is small, the position is pretty close to that of the other objects. A big part of an exercise as I'm doing is to train oneself to know when a sight has been muffed, and to get some feel for good and not-so-good sights, although this consistent deviation to the south is still aggravating me. I note in the data summary above that the sun is perhaps a bit better behaved than the other bodies. With regard to Bruce Stark's mentioning of irradiation, he says that his measurements of solar diameter tend to come out on the large size. Mine come out on the small side, often by about 0.1' of arc, which would move the sun's deviation closer to zero in this case. Another component of my consistent deviation may be deflection from the vertical, as pointed out by Doug Stephen. We live in the middle of the Appalachian Mountains, with the tallest mountains in Virginia about 10 miles away. Although not as massive as the Rockies, they could be an important factor. I welcome Doug's participation. Clearly he has been been very seriously researching the location of his house! I wonder whether he could share with us links to the programs for determining deflection from the vertical. Some of his data comparing the Astra to the Wild also would be a delight to see. I can well imagine that aacurate timing would be critical for the Wild.