A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2020 Feb 4, 07:47 -0800
You wrote: "Just to be clear here, the article does not say that. Implies? Even that's a stretch."
I wondered about writing this as I realized it was probably ripe for misinterpretation. I intended it as the lead in to a reductio ad absurdum argument. In principle the NA could publish tables of lunar distances "as they would appear to an observer in Greenwich, England" as the article expicitly states but that would require you to "clear the observed lunar and then un-clear it to Greenwich" as you describe. Moreover it would introduce unnecessary dependence on the latitude of Greenwich. We might expect to see a bronze marker indicating the official parallel of latitude in the courtyard of the Greenwich observatory but we don't! Using such tables would require more work than calculating lunar distances directly from entries in the NA.
You also wrote:
"These are minor issues. The lack of the word "geocentric" or equivalently "cleared" on the caption of the almanac page is equally minor."
I'm surprised that a comment I made in a post self-described as nit picking could become quite so controversial.
The passage mentioned there is:
"He measured the angular distance between the Moon and the Sun or reference star with a sextant, and recorded the local time. He consulted the British Nautical Almanac to determine at what time the angular distance would be the same if observed from Greenwich."
The article also contains a figure caption:
"This page from the Nautical Almanac shows lunar distances for January 1769 as they would appear to an observer in Greenwich, England"
I don't disagree that the article gets the gist of the lunar distance method right but I have a difficult job seeing that "if observed from Greenwich" and "as they would appear to an observer in Greenwich, England" is anything other than just wrong. In both cases these are geocentric lunar distances and it's hard to imagine why they wouldn't just be described as such. That would be shorter, simpler, unambiguous and above all correct. The concept of the centre of the Earth as a point of reference should be within the grasp of most readers of Sky & Telescope.
You wrote: "Surely the biggest flaw in the article is the numerical one when the author jumps from six minutes of time to six minutes of longitude --equating them!"
Indeed, its huge. Kudos for spotting it. I blipped over the numbers and missed it. However the existence of an even bigger error doesn't make the above statements right.