Astronavigation

The art,  science or craft of finding one's position using a sextant and several books of tables.

A sextant is an optical device for measuring the vertical angle of the sun,  moon,  planets and stars above the horizon (the sea horizon,  not the hills).

It's a difficult and dangerous instrument to use.

Difficult because it requires a degree of knowledge and skill,  first to find the correct heavenly body,  and second accurately to bring the reflected image of that body down to the horizon.

Dangerous because if you get the light filters wrong you'll burn out your retina.   Seriously.   Literally.

In passing,  you might note that a cheap sextant costs twice as much as a decent hand-held GPS instrument.   A good sextant costs 10 to 15 times as much.   It's also 10 times bigger and weighs a lot more.   Astonishingly,  it's more easily damaged by salt water than most GPS sets.

The books of tables  (which list the vertical angles of the heavenly bodies and the exact times of those angles (among many other things)) weigh more than the sextant and cost almost as much.   Some of them need to be renewed every year.

Reading a GPS or a chart-plotter and transferring its information to the paper chart takes less than 30 seconds of time.   Finding a position by astronavigation takes at least 30 minutes,  and might take nearly all day:  when there are no clouds.

The GPS is accurate to within a few metres.   In the hands of a skilled,  experienced navigator astronav is accurate to within a few miles.

It would be reasonable to ask why anyone would bother with astronav.

Midday on Tuesday 3rd February (a bright,  raw day) found Ken and his student  near the beach of Felixstowe town practicing sun sights for a Meridian Passage.

A Master Mariner stopped to chat.   His container ship was being unloaded at Trinity Docks,  a mile or so away, and he,  bored,  had decided on a bracing walk along the promenade.   He was interested because he spent some time on every voyage teaching his cadets how to fix their position from the sun,  moon,  planets and stars.   This,  despite all the complex and expensive electronic equipment on his ship,  much of it in constant communication with complex and expensive satellites telling him exactly where he was all of the time!

Do all mariners,  you ask,  hanker for the days when they steered by the stars?

Of course they do.   If they weren't incurable romantics they wouldn't go to sea in the first place.

Astronavigation is almost our last link with the good ol' days. . . . .more