NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
The Moon is a "nuisance"
From: Frank Reed CT
Date: 2006 Oct 11, 15:24 -0700
From: Frank Reed CT
Date: 2006 Oct 11, 15:24 -0700
In mid-August, Robert Eno started a discussion about the "*&^%$#@ Moon" and how Moon sights seemed to be more prone to error than Sun sights or star sights. Except for a few specific details that imply a little more calculational work with a Moon sight, I tend to agree with everybody who described this as basically an "old navigator's tale" (not you, Robert.. the 'old navigators' who started it). It is essentially a prejudice with little relevance to anyone attempting celestial navigation today. But it sure was a popular prejudice! I found an example in a little navigation manual from 1903: "Latitude by Meridian Altitude of the Moon: The Moon is more or less of a nuisance, and is not used by expert navigators when it can be avoided. The declination changes so rapidly that even minutes of time have to be taken into account, and one is likely to be deceived as to its semi-diameter because of irradiation, which makes the Moon at times look larger than it is. Furthermore, in using the Moon we have to allow for parallax." (--from 'Elements of Navigation' by Henderson, New York, 1903. In his preface, he describes is goal in assembling the book: "Fundamental principles have been explained, but no attempt has been made to elucidate the higher mathematics of the subject. Students who have tried to learn navigation from books like Captain Lecky's 'Wrinkles in Practical Navigation', which is addressed to navigators only, or from Bowditch's 'American Navigator', which is only for mathematicians, will, it is hoped, appreciate this little book.") The idea that "minutes of time" presented a difficulty for the navigator seems strange today, but when you compare with the way a Noon Sun sight was worked back then, it starts to make some sense. Considering the Sun first... Naturally, the Sun's declination changes from day to day and you have to take that into account at Noon to get the latitude right. But the interpolation is easy because the rate of change in the Sun's declination is small. In fact, it was common to use the DR longitude to interpolate the declination, almost as if the chronometer was still a luxury (today we wouldn't even think of doing it that way... GMT is always available, it's ubiquitous, and "the chronometer" is more of an idea than an instrument). By contrast, since the Moon changes its declination rapidly, a navigator would have to be much more careful with the interpolation. The exact minute and second at the time of the observation have to be recorded to get the correct value for the declination. Using exact GMT for a latitude sight must have seemed down-right counter-intuitive for a navigator back then. >From today's navigation perspective (or rather from the perspective of late 20th century celestial navigation), this interpolation is a piece-o-cake, and the tables in the Nautical Almanac are crafted to make the work trivial. If you can't interpolate the Moon's declination, you don't really know how to use the Nautical Almanac. But a century ago, the task must have seemed quite a bit more intimidating. By the way, this business about the Moon being a "nuisance" has nothing to do with lunar distances --just ordinary lunar altitudes. But I suspect that there was some pleasure taken in scoffing at anything "lunar" in this period when lunar distances had finally been declared obsolete and were no longer required for navigators' examinations (a solid fifty years after they had ceased to be generally practical). In other words, I am speculating that disdain for lunar distances, in 1903, may have contributed to a disdain for lunar altitude sights, too. -FER A quick path to the August Moon thread(s): http://www.fer3.com/arc/sort2.aspx?y=200608sd --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---