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    Re: A navigation star list and a great star atlas
    From: Frank Reed
    Date: 2024 May 22, 07:26 -0700

    Charles McElhill, you wrote:
    "I seem to recall there were, for lack of a better term, stars preferred for Lunars. My memory says about 7-9 stars with one not used anymore."

    Yes, exactly. There were nine stars preferred for lunars in the British and American almanacs (and several others that copied the pattern). This is the set selected by Nevil Maskelyne --a revision of that ealier list from 1764 that I posted a few days ago. The "new nine" are shown in the attached star chart, which is centered on the ecliptic. The ecliptic, of course, is the path of the Sun through the heavens, and it's also rather close to the path the Moon takes in its monthly journey around the celestial sphere. The easy choices for stars appropriate for finding GMT/UT by lunars are Aldebaran, Pollux, Regulus, Spica, Antares. They're bright, and they're close to the ecliptic. That set gets us about 60% of the way around the sky. But then we encounter a wasteland...

    There are no bright stars at all through Capricornus, Aquarius, Pisces, and Aries. Even experienced stargazers have trouble finding the stars of those constellations... Maskelyne added two bright stars well off the ecliptic in that part of the sky: Altair and Fomalhaut, which are useable when the distances are somewhat larger. He also added two fainter stars: Markab and Hamal. And for a dozen years, there was one more, beta Capricorni, now officially known (IAU official) as Dabih. I've circled that one in dark grey on the attached star chart (in about the same ecliptic longitude as Altair but much closer to the ecliptic). Your question inspired me to have a look and get the final date for Dabih. It was listed in the Nautical Almanac lunar distance tables from 1767 through 1778 --twelve years. I did not find any note from Maskelyne in the introductions to the almanacs in those years admitting that beta Cap was a dud, much like alpha Cap before it, which he had included in his 1764 list.

    We like ecliptic stars for lunars only if we are using them for determining time (that's absolute time, GMT or its modern cousin, UT, in other words). The Moon's motion is nearly along or parallel to the ecliptic, and it directly approaches (or recedes from) stars that are close to the ecliptic. That's why they're effective for determining GMT --the lunar distance angles to those stars near the ecliptic changes at a rate close to the maximal rate on any date. This is also true for the Sun, which was the most important object for lunars. It's true for the planets, too, but they were not listed until decades later.

    In the modern world, navigators have no need to determine absolute time by lunars. Even in creative "what if" scenarios, there is almost always a better way. But lunars serve as a link to maritime history, and they're fun, and on practical grounds there is no better test of the navigator and the sextant. It's also possible to get an approximate position fix (latitude and longitude) using lunars when UT is given that completely eliminate the horizon from the observation. For these modern applications of lunars, other stars are just fine and can be selected for convenience.

    I'll throw in a reminder: my lunars workshop is live, in-person at Mystic Seaport Museum in less than two weeks, June 1-2. It's possible that this will be the last in-person lunars class for several years. Get it while it lasts... Details and registration: https://www.reednavigation.com/lunars-class/. If you're a member of Mystic Seaport, you can register with a discount at https://mysticseaport.org/.

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA


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