Astronav Update: Mistakes were Made

May 10, 2019

Day 217

Noon Position: 19 28N  54 27w

Course(t)/Speed(kts): NxW 3.8

Wind(t/tws): WNW 4

Sea(t/ft): —

Sky: Squalls on al sides

10ths Cloud Cover: 5

Bar(mb): 1017

Cabin Temp(f): 86

Water Temp(f): 90

Relative Humidity(%): 64

Sail: Big genoa and main, reaching on port

Noon-to-Noon Miles Made Good (nm): 69

Miles since departure: 29,277

Avg. Miles/Day: 135

Leg North Miles: 6,349

Leg North Days: 51

Avg. Miles/Day: 124

Most of the night we made between two and three knots. The sky started clear, but by morning, Mo was walled-in by heavy, black squalls. Nothing moved for several hours. Only when windlessness holds one prisoner in this way does such an intensity of quiet feel oppressive.

Then, just after noon, the squalls melted away and a cool, north wind filled in. On that we now make six knots on an empty sea under an empty sky.

—

I chose a challenging week on which to darken the chart plotter, and then I committed several bone-head blunders.

Firstly, we were passing under the sun on the first couple of days of this exercise. This phrase, “passing under the sun,” indicates that ship latitude and the sun’s declination (the celestial word for latitude) are converging. Ship and sun are nearly on the same plane.

In this case, when I switched off the plotter display, our latitude was just over 17N and the sun’s declination was approaching 17N.

The effect is that the day’s sun shots, which are attempts to triangulate your position from the sun’s different positions throughout the day, don’t produce a nice triangle of intersecting, you-are-here lines, what sailors call a cocked hat. Rather, the lines run nearly parallel to each other.

This does’t mean one’s results are wrong, they just appear less precise, and as such, they do not add to one’s confidence in his work.

Then I moved my watch up to GMT+4 and set the minute hand just slightly askew of its mark. That night’s star shots were a mess. A four second error in time keeping causes error of a mile in the results; so, you can imagine what being off sixty seconds will do.

Then I bonked the horizon mirror coming on deck and didn’t catch that I’d caused a two minute index error until the next day.

But we’ve worked through the bugs and have remained relatively sure of our position.

The most difficult day so far was yesterday. Recall that this is largely an exercise in dead reckoning (staying aware of ship position between fixes via compass and log), and you can imagine what light, fluky, ever changing wind will do. Now our course is northwest; now north; now west; now becalmed but moving with current. After a few hours of this, I’m lost. So it was gratifying to find that today’s dead reckoning was not too badly adrift from the noon fix.

—

Much of learning astronav is learning how to deal with error, because there is so much opportunity for it in the many steps from sight to fix. A three-shot sun fix (morning, noon, afternoon) requires roughly 60 separate actions; not one is the least bit difficult, but they are legion.

At first, you hedge your bets by taking multiple shots; if one doesn’t seem to be working out, move on to the next. But after a time you become confident in your ability to shoot a heavenly body (especially in middle latitude conditions), and so you take but one. If that shot fails, you backtrack into your calculations for the mistake.

Most errors are simple. Did you carry the one? Did you add rather than subtract? Some errors in time keeping are easy to remedy. For example, a shot that is out by around 15 miles is likely due to grabbing the wrong minute of time from the watch. Often if my corrected altitude and computed altitude aren’t close, I’ll scan the Sight Reduction Tables page for a computed altitude that is close and take its corresponding Local Hour Angle back into my computation. This often shows I’ve picked up the wrong data from the Almanac.

You also become fiendishly focused on accuracy. When recording an altitude, I often talk to myself as I read the result off the vernier. “That’s 34 degrees, not 35; not 39; that’s 45 minutes, not 35; not 40,” etc.

—

When it all comes together, it’s easy to feel The Great Navigator.

Then one must recall that there are more than a few people reading these reports who had to learn celestial to go off shore … because when they went exploring, there was no other option. In one generation, proven methods of way finding in use for hundreds of years, have been largely forgotten.