Sea change

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For a while there it looked as if Peter Nichols was doing everything right. He had married his sweetheart, and together they were living a poor, but happy, sailor’s dream in the Caribbean. Delivering yachts, running boat charters, and finally buying a boat of their own. Compared to all the others with similar dreams, they were making great progress.
Increasing the range of their adventures they decided to sail their 27-foot wooden boat Toad across to Europe. Never mind that Toad was engineless, or that it had been built in 1939, and that the bottom had been sheathed in a nylon cloth. With the apparent blessings of Poseidon, they arrived safely in England and spent the following months sailing and watching their marriage fall apart.

A year later, Nichols decided to get on with his life and take Toad to Maine where he knew there was always someone looking for an old wooden boat. He is a competent celestial navigator and has worked on every part of his boat. The fact that he doesn’t have an SSB radio, a GPS, an engine, a desalinator, or any of the other million items long-distance voyagers believe they must have for their survival, doesn’t faze him.

He was a man of courage for going to sea single-handed surrounded by the memories intangible, and otherwise, of his former wedded state. Aboard Toad were diaries written by his former wife recounting her version of the events they shared. Nichols had never looked at these before and he read through them alone on the Atlantic. A strong man indeed!
As circumstances would have it, west of the Azores, things begin unraveling. I won’t say more than that for I encourage readers to seek out the book called Sea Change that Nichols wrote of his experiences. Nichols has since written two other books and is teaching at Bowdoin College in Maine.

As for the navigation problem, we have Nichols getting a fix from a moon and a sun sight. The moon sight is not as difficult to reduce as it once was but there are plenty of opportunities of making mistakes so the key is to pay attention.
The day in question is July 8. We will, as usual, use the 2008 Nautical Almanac, Toad is at a DR of 32° 05’ N, 42° 47’ W. The time is in GMT. The height of eye is 10 feet and there is no index error, nor any time error. The first shot of the lower limb of the moon is done at 17:51:25. The Hs is 45° 38.2’. The lower limb of the sun is shot at 17:55:30, and the Hs is 49° 6.9’.

Reduce the shots and calculate intercepts, then plot LOP’s and locate fix position.

Moon shot
A. What is Ho?
B. What is intercept?
Sun shot
C. What is Ho?
D. What is intercept?
E. What is fix?


A: Ho is 46° 28.9’
B: Intercept is 2.9 nm toward
C: Ho is 49° 19.0’
D: Intercept is 11 nm toward
E: Fix is at 32° 06’ N by 42° 50’ W

By David Berson
This problem gives us the opportunity to test our abilities! Reducing a fix from a moon/sun LOP challenges our understanding of the concepts of celestial navigation, as well as tests our patience as we wade through the tables. To be honest, the moon shot now is so simple that even I can do it. Once upon a time though it was the bane of celestial navigators — not so anymore. The moon is a great object to shoot; it can provide an easy target and along with a sun sight can create a dynamite fix. I’m sure most of you are using a celestial calculator to solve these problems, but if you have the courage to go to the Nautical Almanac, I guarantee that you will feel really good about yourself when you exit this seeming maze of numbers. Onward!
The day is July 8. DR is 32° 05’ N by 42° 47’ W. Height of Eye is 10 feet. The Lower Limb of the moon is shot at 17:51:25 GMT. The Hs is 45° 38.2’. The Lower Limb of the sun is shot at 17:55:30 GMT and the Hs is 496.9’
What is Moon HO
GHA 17 hrs         3° 37.3’     v+15.1     Dec S 3° 01.0’  d +14.7’   HP 56.4
Inc& Corr           12° 16.1’                      +           12.6’    
+ v corr                      13.0                    Dec S 3° 13.6’
GHA                    16° 06.4’
+360°                 376° 06.4’
-Ass Long            43° 06.4’
LHA                  333°
NOTE: v corr is always added to GHA in moon sights. D corr is either added or subtracted from declination dependent on the increase or decrease of declination.
360° was added to the GHA so that we could properly subtract the assumed longitude and not get a negative number. The addition of a 360° is the same as adding 0°! 
Hs      45° 38,2’
-dip           03.1’
Ha       45° 35.1’
+                50.1’
+                03.8’
Ho        46° 29.0’
The corrections are obtained from the moon tables at the rear of the NA. The explanations for the use of the tables are clear if you reread them a few times!
Go to HO 249 Vol. 2
Hc      46° 37’   d –49’        Z 139°    
Table 5 for 14’ of declination = 11’
Hc 46° 37’
–     11’
 Hc 46° 26’
Intercept:   Ho 46° 29.0’
-Hc 46° 26.0’
 INT.       3 nm Toward
Sun Sight: GHA   17 hr     7342.4’
Inc & corr                            1352.5’
GHA                                      8734.9’
-Ass Long                               4234.9’
LHA                                         45
Hs   496.9’
-dip       3.1’
Ha    493.8’
3rd corr + 15.2’
Ho     4919.0’
Vol 2 Ho 249 Ass Lat 32 N
Hc 4859’                   d+24                    Z 93 convert to Zn 267
Table 5 +9’
Hc 4908’
Ho 4919’
-Hc 4908’
Int 11nm Toward.
In both reductions the ass latitude is 32N. This is for use in the plotting. When plotted the moon/sun fix is 3206’N by 4250’W. If you have gone this far get up and a drink. I know I am!
By Ocean Navigator