A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2020 Jan 25, 18:25 -0800
After an interesting morning's work I have deduced that a suitable AP location for me would be > 22°S lat <52°S and long - 174°E. I do not have an orange but an old globe proved handy. It has scratches on it from George, a cat my wife had many decades ago. I am on terra firma and the globe suggests Stewart Island, Norfolk Island, Auckland Island, the Antipodes or Matiu/Somes island would be good starting APs. Any other large land mass in the area would also be suitable.
Later today I plan to perform a long by chron when the sun is on the prime vertical to confirm the longitude. It should be an easy sight as I need not worry about index error - I will be working to the nearest degree.
I use an AH so it will be later in the year before I can use a noon sight to get a better estimate of the latitude. If I can obtain a better compass then a reverse entry into a table such as Burdwood may give me a better estimate of latitude.
Firstly what I know;
(a) The sun is rising. My phone comapass is sufficiently accurate to tell me where north is. Hence I am in the southern hemisphere and lat is S.
(b) The solstice was a month or so ago so the sun's dec is approximately 22°.
(c) At noon the sun's alt is > 60° so will assume lat and dec are of same names. Therefore dec = 22°S. Thiis is confirmed by the weather patterns although global warming may soon change this.
(d) I use an AH so know from observation that the sun's alt at noon is >60°. At noon lat = 90 - Ho +- 22 from which Lat <52°S.
(e) I have observed the rising sun crossing the prime vertical. From Bowditch (1959) table 25 I learn that the lat must be >22° for the sun to cross the prime vertical.
(f) I am on terra firma.
Conclusion A: >22°S Lat <52°S
At this point I planned to take a bearing of the sun and estimate local apparent time by a reverse entry into Burdwood's azimuth tables. Unfortunately the compass was not accurate enough so I abandoned this method.
From previous playing with a compass I know I know which point on an adjacent hill is true north. I will observe the shadow in my AH to determine the approx UT of noon from which I can compute the approximate longitude.
Sun true north at 0035 2020/1/26 UT
EOT for 26th Jan = subtract 12 min. This is an approcimation as I am using a 1932 edition of Inmans tables.
Hence apparant time at Greenwhich is 0023.
GHA = 1223
From a time to arc table in Raper (1906) 1223 min = 186°
GHA = LHA + Long +-360
LHA = 0 so long = GHA +-360
From which long = 174°
From previous playing with a compass I also know the a neighbour's chimney is due west. I will perform a long by chron. to determine the approx longitude.
sin Z = sin t cos dec sec h
Z=0 so sin t = sec dec cos h
Because of the precision I am working to I will not need to worry about index error.