Astronavigation

Monday 2 February

The journey from Cambridge to Levington, mostly along the A14, takes about an hour and a quarter: if traffic is heavy perhaps an hour and a half. If a lorry crash has blocked both lanes, then it's three hours.

The course was held up for an hour: there was only one candidate; held up by the accident.

Then the day began to improve.

Ken spoke easily through the syllabus, and then through the principles behind astronavigation. Cups of tea and "chat breaks" were plentiful and the day progressed swiftly and happily.
The principles understood, the arithmetic was laid out on proforma sheets (work flow-sheets almost) in a clear and systematic way.
Example Sun-sights and Meridian Pass sights were translated into Position Lines and Latitude on the chart.

Tuesday 3 February

As a break from the tables and the arithmetic and the worksheets, Ken took the sextant from its case and explained the detailed principles and corrections.

Then into his car for a run to Felixstowe promenade. The sextant, GPS, notebook & pencil and watch were laid out on the public bench and the student began to look for the sun.
Big shiny yellow thing? in the sky? more or less South? several handspans up from the horizon?
Now find its reflection and bring it down to the horizon.

Back in the classroom the noon altitude was corrected and noted. An altitude was calculated for the Assumed Position and a Latitude plotted on the chart. To everyone's joy it was within 1nM of the known position!

On a tide of euphoria the afternoon was spent working exercises from the course booklet.

Wednesday 4 February

The tide of euphoria having ebbed, the sight reductions became tedious. The beach of effort was littered with the pebbles of arithmetic errors, mud from the wrong tables and a quicksand of eraser rubbings. As light relief, Ken offered short voyages of Meteorology, Passage Planning and Ocean Currents, but by mid-afternoon the student had foundered, head in hands, pencil and spirit broken.

The journey back to Cambridge was miserable. It was dark and it snowed. Other drivers, and his misery, bullied him. His last sandwich, saved 'specially from lunch, shed its contents onto the seat between his legs.

Another effort, before dinner, was blighted by a single arithmetic error, easily spotted and corrected.
A final effort, after dinner, was rewarded with no arithmetical errors, no fumbling for the correct table, a tiny Intercept and a correct answer.

O, joyous rest: O, blissful sleep!

Thursday 5 February

Tea and questions, and then the Assessment.

One arithmetic error in one sight reduction, spotted and corrected.
Easy answers for the Meteorology and the document planning, and it was done.

Why the fuss? What fuss?

Sun sights understood, the planets were easy. A few extra corrections, to be sure, and Venus (bless her) had an extra surprise.
Victualling and passage planning were straightforward and there was even time for another Meridian Passage on Felixstowe promenade.

On the long drive home Venus hung over Cambridge, beguiling and taunting, promising undreamed-of navigational accuracy.
Later, the moon rose nearly full over the Eastern horizon. One day she, too, would reveal a position line on a chart.

Friday 6 February

Sextant moon sights are not easy, she moves so swiftly and smoothly. Once caught, she reduces to a position line and intercept as easily as any of the planets, but the lack of accuracy reflects the difficulty of the chase.

Then the spherical trigonometry, the magic of the PZX triangle, where some say that parallel lines do meet at a point and curved lines can be at right angles to one another. The final expression of Euclidean geometry.
At last, a break from the tedium of tables, worksheets and sight reductions.
The simplicity of a set of clear equations into which a man's calculator could sink its electronic teeth and feed his brain.

You have to ask.
Given the choice between a week of Astronavigation and a week on a boat, which would you prefer?
Tough question.

John Starkie

February 2015