2 June – the joys of celestial navigation

20h00 UTC

There is a good reason that GPS was invented. It’s to stop amateurs like me from throwing sextants, nautical almanacs or chronometers (or possibly all of them) overboard in frustration when trying to shoot an unwilling heavenly body!

This afternoon, about half an hour before sunset, it looked like I would get a good clear sky around twilight to take a couple of star sights during the comparatively short twilight available near the tropics. So I dashed down to the chart table, all of two paces away, hauled out my nautical almanac and started to prepare. This involves narrowing down the time of civil twilight by checking the time of twilight at Greenwich and adjusting it for my longitude. Then from a few relatively simple calculations, I am able to refer to the Sight Reduction Tables for Air Navigation, Selected Stars Epoch 2020-2024 and in a neat table, there is a list of seven stars best suited for my latitude and longitude, together with their Azimuth and declinations to make it easy to identify them. All that is required from there is to draw up a worksheet so that the time and observed altitude of each star can be recorded for easy reduction to a line of position on a piece of graph paper. Successful completion of these steps gives a very accurate fix. All that is needed is a clear view of the stars and a visible horizon.

With the sun having set and five minutes to go before the stars should theoretically become bright enough to identify in the darkening sky, I sit behind my spray dodger, sextant in hand with the declination of the first star, SPICA, preloaded. And then the clouds roll in from the south. Obliterating that option. But all is not lost, I have stars to the East, west and north east to shoot. Reload the sextant, only to have exactly the same thing happen. In all directions. Twilight is short in these latitudes, but there’s a sliver of moon in the west keeping the horizon more or less visible so I wait patiently.

Eventually I had to call it a night as I couldn’t get a clear shot of any star, let alone the ones I’d identified. So back to the office and pack the sextant away, tidy up the chart table and make a cup of hot chocolate to compensate. When I step back into the cockpit, I’m greeted by a magnificent display of stars from horizon to horizon and not a cloud to be seen.

Fortunately, a month at sea has created a calm demeanor and not even the mug of hot chocolate was harmed!

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