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    Re: David Thomson and his lunar tables
    From: Antoine Couëtte
    Date: 2010 Mar 20, 01:49 -0700

    Dear Frank,


    In [NavList 12359], you published a very interesting "real world" example of a Sep 1809 Lunar on-board Brig-Reaper of Boston.

    *******

    I replied with the following results in [NavList 12366] + a clarification on the date in [NavList 12371]:

    ...///...

    and with to-day computing tools, we get an observation date of Sep 15, 1809 ...///... as we would reckon it to-day, and ...///... a UT time of 09h41m32.9s with an observed position as follows:

    S 27°47'6 and E 048°34'8 for an account longitude equal to 48°40' (he found longitude equal to E048°45'). So this sharp Observer was definitely within 5 minutes in Longitude from his DR.... Whaooo !!!

    Some more examples ? :-)) ....

    ...///...

    *******

    Meanwhile, I crossed checked my results with your On-Line Computer just to find a 0.0' difference in Longitude for the given time and full coordinates. Once again, we are on-board the very same ship . :-))

    By curiosity, I looked up your enclosed Log-book Tabular clearing computation to try guessing where the Navigator did take his (DR) Latitude in account.

    His DR Latitude was not far from S27°47'6. So, let us do a bit of numbers guessing :

    Lat = - 27°47'6 = - 27.7933 , Lat * 2 = - 55.58667 , Lat / 2 = - 13.896667

    I fail to see any such number on the Log report.

    Logaritms then ? But since they are not defined for negative numbers,

    - Did they use the Lat Cosine (= 0.8846352) , with has 9.9467642 for 10Log ? But in such case "some" information is lost, namely the North or South Hemisphere information since (Cos -A = Cos +A ), or

    - Did they use some kind of special function for angles such as versine/haversine to avoid all problems with Logaritms ???

    I am just curious here because - just from looking up at this clearing computation method - I fail to see where and how he managed to take their (DR) Latitude in account in their Moon Clearing process.

    I would think that some knowledge of the Observer's Latitude is an information absolutely required in any Lunar Clearing process, whatever the method actually used, would it not ?

    And by the way, which Lunar reduction method is he using ? You said that it is not the one recommended by Bowditch.

    Just a "ribbit" question from a Curious ...

    Kermit


    Antoine M. "Kermit" Couëtte
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