Sunday 10 November 2013

Day 34: Navigation Practice


Today dawned filled with thick clouds. In fact, when I came on watch this morning at six, a heavy squall had just finished washing us clean of the salt from the last few days.

Pity I wasn't awake; I could have had a free fresh water shower!

I was a wee bit worried, as I had planned some navigation exercise for myself for today. A noon sight as well as a three point fix using the stars at dusk twilight.

The noon sight almost did not happen, as a huge, thick cloud obscured the sun just two minutes before noon. Luckily the cloud passed and I had a clear sun over the LAN (Local Apparent Noon for my non-sailor friends). Measuring the height of the sun at this time reduces directly to a latitude measurement. This makes life somewhat easier for the navigator.

I specifically did this, as it represents a different measurement than what I had been using for the voyage thus far. It was also high time to break the monotony of the daily sun sight and running fix calculations and do something else.

The rest of the "something else" work relates to star sights. There is quite a bit of preparation going into star sights. Firstly, you need to predict your dead reckoning position for the time of civil twilight. More buzz words, I suppose.

For most of the time, star sights are only possible during the time of twilight, when the sun's centre is between three and nine degrees below the horizon. Earlier than that, and the stars are not visible. Later than that and the horizon is not visible enough. It may be visible, until you have a star's light in your eye as well, thereby blinding you to the faintly defined horizon. You need a well defined horizon, with good contrast for a decent measurement.

Been there, tried that.

Stick with what the book tells you to do. Six hundred years of ocean navigation theory condensed in a book counts for something after all, I guess. Nothing new here, just solid practical procedures.

The calculations are quite interesting, as you need to keep a clear head on exactly what time measurement you are working with, be it local time zone, local longitude time or UTC.(GMT). All three these time units come into the equation to find out when twilight will be. And, oh yes, just to complicate the issue, there is civil twilight and nautical twilight. Different angles of the sun below the horizon.

You need to check this time of twilight, then calculate where you will be at the time, then referencing which stars will be visible and suitably spaced for a three point fix. With my rudimentary knowledge of astronomy, the sight reduction tables and the nautical almanac are heavensent aids. I look up the relevant stars for my predicted position, then check in the star chart where they are situated in the sky.

Of course, the star charts are not oriented in a "heading up" way, so I have to orient myself on the chart with reference to stars that I do know. Luckily, there have been novices, ham-fisted tyros and other types of people struggling doing stargazing, so most of the useful stars are arranged in the signs of the zodiac. This makes it a lot easier.

However, I am quite new to the northern hemisphere, so now I have to learn a whole new set of stars positions and their names. The nice part about this is that the images in the zodiac are the right way up viewed from the northern hemisphere where all the astronomers of old used to live.

Having done all the required preparation, now came the time for the sights. I just hoped and prayed that the clouds would allow me a set of sights. Then I shall know where I am on this earth.

Without the benefit of electronic instruments.

But Mother Nature is always in charge, irrespective of the amount of preparation and planning that you go into. Too much haze in the air, never saw a star until it was quite dark. Then I could not see the horizon. Just like the text book says.

At least I got a sight on Venus.

And I now should also be able to find the stars Alpheratz, Aldebaran and Achernar again.

Perhaps a good lesson to us all. The words of the Ecclesiasticus, at the end of his book, in his parting message, come to mind: There is no end to the making of books.

It is now time to take a deep breath, relax and just enjoy the expanse of the night sky above me in quiet solitude on my watch.




Authored by Johan Zietsman


Last updated on 2013-10-28

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