A Note on Celestial Navigation and a Frigate Bird

Day 159/37

Noon Position: 06 58S 154 05W

Course/Speed: N5-6

Wind: E15-18 (20 – 25 all afternoon)

Bar: 1011, falling

Sea: E5

Sky: Cumulus to 20%

Cabin Temperature: 89

Water Temperature: 84

Sail: #2 jib full; main one reef, close reaching (double reefs in the afternoon)

Noon-to-Noon Miles Made Good: 137

Miles this leg: 4,512

Avg. Miles this leg: 122

Miles since departure: 21,619

Aeolus is a trickster god, it should be admitted. On the 28th day, he created the The Trade Winds, saying, “Blow trade will these winds. Now go forth and multiply.” And they do blow trade. But man, do they shift in velocity something fierce. I’ve reefed and un-reefed thrice today, which I think must entertain Aeolus. I sure hope he’s watching.

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That tired complaint to one side, it’s been a fine day on Moli, for today my sun sights yielded an accurate fix.

If find celestial navigation to be something like the opposite of going to the dentist; it’s fun to do; but the procedure is eminently forgettable. Each time I go back at it (I did sun sights all the way down the Pacific in November and December of last year–and not since), it usually takes me three or so days of sights to work out the kinks. This time it was *just* three days.

What I know I’ve learned from Tom Cunliffe’s little book titled simply Celestial Navigation, and the method I’m most comfortable with goes by the name “Sun-Run-Sun.” In brief, it requires three shots of the sun throughout the day, one mid morning and another mid afternoon to fix lines of position that include longitude, and a noon shot line of position for latitude. The “run” refers to the running fix in pilotage, only here one is running up to noon and back to noon the morning and afternoon lines of position from the sun.

Beyond the taking of the sights themselves, there are only six steps to the computation, but these require eleven look-ups. By “look-ups” I mean eleven bits of additional information needed to make the computation possible–seven in the Almanac and four in the Sight Reduction Tables.

I use the Nautical Almanac produced by the UK Hydrographic office and Sight Reduction Tables for Air Navigation, known more broadly as HO 249. (These were produced for WWII bomber navigators; they are simpler and quicker to use than the Navy’s HO 229, and they are wee bit less accurate–a fair trade on a bounding boat.) Having got all one’s ducks in a row, the math is simple addition, subtraction and division. No calculator needed.

But holy cow, are there ever opportunities for error. Wrong date, inaccurate read of the time, bad conversion of local time to GMT; bad sight taking, bad reading of the altitude off the sextant; wrong page in the book(s), number transposition, sloppy conversion of base 100 to base 60 (when finding local noon); failure to remember that most addition/subtraction is base 60 (it is time, after all); wrong latitude name in the sight reduction tables; wrong column; wrong column, wrong column.

Then there’s taking the sight itself. The boat is acting like a washing machine that’s just swallowed an elephant, this while you are trying to get an altitude accurate to the second off a body so far away it takes eight minutes for light to travel from it to you.

But today, success.

Local Noon position by

GPS: 06 56S 154 04W

SEXTANT: 06 57S 154 07W

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Someone on the site asked how often I use my sextant. I think the answer is “not often enough.” It’s immensely rewarding to find one’s position on the globe the “old way” (the *only* way until recent decades).  And once the procedure is habit, it’s not hard to keep it up. But when the going gets rough, it’s the first thing to fall away…because it can. The chart plotter offers a position to the 100ths of a second every second of the day. And I don’t even have to push a button.

This is one reason I admire the sailors in the Golden Globe Race that commences in a month or two. Those singlehanders, departing from Plymouth for a non-stop lap of the world via the Southern Ocean, will find their position by sextant or not at all.

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First lone frigate bird sighting today. Easy to identify. They are the only pelagic that soars high up with the clouds.