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    Blackburn's C2 table...a ramble
    From: David C
    Date: 2016 Dec 2, 23:17 -0800

    This is a bit of a ramble showing how one thing leads to another.............

    While searching newspapers for photos of sextants I came across the following June 1914 article:


    The cynic in me said that the real reason that Captain Blackburn wanted Captain Norton's letter posted was because it would help the sales of Blackburn's book (-;

    Then I noted Norton's comment about taking sights of Canopus and Sirius and working them as ex-Meridians. This was at 5 30pm which  assume is local time. In 1914 NZ was 11 1/2 hours ahead of GMT so LAN would have been sometime between 1200 and 1230 local time. Did Captain Norton really  do two ex-meridians five hours after LAN?

    Norton mentioned using Blackburn's "ingenious Table C2". I have a 1923 edition of Blackburn's  Tables for Azimuths, Great Circle sailing and Reduction to the Meridian. These tables contain the C2 tables (attached). Studying the explanation I concluded that the C2 table allows Sumners (and presumably intercepts) to be plotted on a plane rather than mercator chart. It is based on the A B C tables.

    A+-B -> C -> azimuth on a mercator chart

    A + B -> C -> C2 -> azimuth on a plane chart so a page in a workbook will do.

    Getting back to Captain Norton..........  The inference is that the Canopus and Sirius sights were plotted as Sumners.

    Glancing through the text in Blackburn's book I conclude that at the beginning of the 20th century Sumners and Mark St Hilaire methods were used interchangeably. Sometimes one sight would be worked as a Sumner, the other as an intercept. Why I do not know.

    I have a pdf version of Cotter's A History of Nautical Astronomy. Puzzled about the 5 30pm ex-meridians I scanned the chapter on finding latitude. I discovered that (in theory) latitude can be found when the sun is on the prime vertical. The time t seconds (time)) for the sun to rise a seconds (arc) give latitude thus:

    cos lat  =  1/15   x  a/t

    I have the following sights:

    085443   33° 47.6'

    090323 35° 24.1'

    Delta t   520 sec

    Delta alt 96.5'

    I suspected that the 8min 40sec interval was too large but decided to give it a try.

    from which lat =  42°  4' ( gps 41° 06.50)

    I am not suggesting that Captain Norton used this method - as I said this as a ramble.

    Cotter suggests that the time for the sun to move one semi diameter be measured:

    t = a / 15/cos lat = 16*60/15/cos 41  =  85 sec

    Therefore next time I take a sight when the sun is on the prime vertical I will take additional sights at about one minute intervals.

    Those who just study intercepts and 229 do not know what they are missing! That's enough for now (-;


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