A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Feb 2, 18:53 -0800
Robin Stuart, you wrote:
"It would make little sense to do the extra work and convert to topocentric lunar distances for Greenwich, England as the article implies."
Just to be clear here, the article does not say that. Implies? Even that's a stretch. If a reader misinterprets a caption to the image of the almanac page after reading the brief paragraphs on lunars in the article, which are very basic, that reader might —possibly— draw that mistaken conclusion. But it's not as if there's no other way of interpreting what's written. The article most certainly does not say anywhere that you have to clear the observed lunar and then un-clear it to Greenwich, or otherwise calculate topocentric Greenwich numbers.
I would add that I re-read the article and its various captions three times tonight to see if I somehow missed something. And it's just not there. I do agree that the caption for the almanac page is a bit weak, could be better. But that's an issue with other images and captions in the article. For example, Cook in a caption is described as "stubborn" and that's why he was killed in Hawaii? Stubborn?? Not racist? Not prone to rage? Just stubborn. Sure... Later the map of Cook's voyages notes that the return voyage after Hawaii is dotted (dashed) after his death. Hey, it looks like he came back to life when they crossed back into the Atlantic! And the caption of the dust-collector sextant oddly mentions the US Naval Academy's change of instruction in 2015, which has no relevance to that image. These are minor issues. The lack of the word "geocentric" or equivalently "cleared" on the caption of the almanac page is equally minor. It might have been even better if they had noted that the times on the almanac are Greenwich Apparent Time rather than GMT. That might actually have generated some thinking. The principle of lunars as expressed in the article is clear, and only a perverse reading would lead anyone to think you had to reverse the clearing (to get the topocentric distances at Greenwich) in order to compare against the tabulated values.
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! This is wrong by more than an order of magnitude. The author is hardly the first to make that mistake, but it's an embarrassing error that someone should have caught. Many astronomical software developers get caught on the difference between minutes of RA and minutes of Dec, for example, when doing proper motion calculations. It's the same factor of 15. Is that a minor error or a serious error? In an article like this, it may not matter too much. But there are enough readers of Sky & Telescope who might be tempted to think through the numbers on that one that it could be a concern. I'm a little surprised that it wasn't caught during editing, but given that it's not an article where the numbers are primary, it probably was passed over without a thought.