A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2018 Dec 15, 18:12 -0800
Yesterday afternoon I checked a sun app to determine how near the sun was to
the southern solstice. There was just 11' to go. Then I realised that in just
20 minutes the sun would be due west. It was a perfect day for a sun sight with
a clear blue sky and little wind to disturb the artificial horizon. A ideal
moment to do a long by chron.
Usually when I use my sextant I take lots of practise sights first. My eyes
become tired and I find it hard to take the sight and then read the
vernier. I decided to take just two sights. Five minutes before the sun was due
west I took a test sight to make sure everything was set up correctly.
Then at 1716 NZST I took the sight that mattered. I finished by
measuring the index error.
First the IE maesurement
from which IE = 4.2' on
Sun's SD = 16.4'
From the Aeronautical Almanac SD = 16.2' which confirms that I am using
the sextant correctly.
Measurements were made with an AH. I am pretending that I do not know my longitude and my latitude is
15th Dec 2018 04 16 08 UT
GHA = 243° 45.7' + 1 ° 32' = 245° 17.7'
Dec -23° 15.4'
Ho 36° 58.3'
I used an fx-82AU calculator and the formula
cos t = (sin(Ho) - sin(lat)*sin(dec))/cos(lat)/cos(dec)
To eliminate the need to think All Science Teachers Count and work out if the - sign should be a + I entered lat and dec as signed values.
lat -40° t 60° 24' 7"
I then realised that if I calculated long for at least one more lat I would be
doing a Sumner:
lat -41° t 60° 24' 40"
lat -42° t 60° 24' 10"
long = t - gha + 360 so
lat -40° long 175° 06.5'
lat -41° long 175° 07'
lat -42° long 175° 06.5'
This demonstrates why an accurate lat is not important for a long by chron if the sun is due west or east.
For the record my gps long is 175° 05.2' Given the relatvely poor optics of my
sextant, the difficulty of reading a vernier, the use of an AH and my spur of
the moment decision to perform a long by chron I am happy to have determined
my long to within 2' (1.5 nm at my latitude). The final part of my journey home can be by paper map and pilotage.
The hardest part of the exercise was correctly typing the formula into the
calculator. I kept getting an Error message whenever I pressed the = key. It
would have been so much easier if Blackburne had calculated his hour angle tables up to lat
I suppose that if I wanted to follow tradition I could get my copy of Nories
down from the bookshelf and calculate t using trig, log trig and anti log
tables but there are limits to how fay I will go to avoid using electronic