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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:

https://paperspast.natlib.govt.nz/newspapers/EP19140601.2.19

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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