NOON SIGHTS ON COASTAL PASSAGES for FUN
Do you have a sextant on board?
We usually associate astronav with deep sea passages, but there are things you can usefully do on coastal
passages, even if it is only for fun or practising taking your sights.
The most useful sight you can take on coastal passages is the altitude of the sun at meridian passage, which
can give your latitude, usually to within a mile or two on a calm day, and if you plan it well your longitude also
to within a few miles, if you’re lucky. If you add a mid-morning and mid-afternoon sight too, you can do a SunRun-Sun calculation if you have a log, and know the tides [or better still use the GPS to tell you course &
distance run over the ground! I did say it was for fun!]
You need to be a fair distance from land in order to have a horizon on the sea rather than on the beach! The
sea horizon can be approximately calculated from the formula
Distance to horizon in nautical miles (nm) = 1.1 x square root of [height of eye(HoE)] in feet.
On the internet you will find calculators to work this out properly 1 and these tell you that the correct distance is
for HoE 7.5ft = 2.9nm; 8ft = 3.0nm; 9ft = 3.2nm; 16ft = 4.3nm.
From the chart using dividers you can mark off the distance to the horizon from the Low Water mark and that
will show you how far you have to be out to sea in order to have a sea horizon towards land. On a modest size
yacht your HoE is unlikely to be much more than 16 ft so if you are more than 5nm from shore you should
have a sea horizon to work with, even if it does have a land-background. That is not always such a bad thing!
The sun is against a dark background; but beware of mirage refraction effects.
How to measure your HoE? Well, while in harbour stand where you normally would take a sight from, often
tied to the mast or in the cockpit, then use a (straight) boathook held at eye height and horizontal. With a
plumb line tied to the end, adjust until it touches the water; then measure the length of the string.
Taking the noon sight is quite straightforward when you have practised a bit. You can practise at home, at
least the act of bringing the sun down to a horizontal line and measuring the altitude as it goes through the
zenith. The actual values will mean little as it is not a true horizon, but the practice and observing what happens are important. I use the garden fence as the horizon, which is not very good but it works after a fashion;
you would be better off with a distant flat hill nearby if you cannot see the sea.
There are lots of websites to help you2 and some good books3. There is also even a Nautical Almanac on line
with pages that will permit you to do Sun meridian passage calculations 4. Those with ipads, tablets etc on
board should be able to dial these up from a few miles off-shore but you’re better off preparing them when on
shore.
Time is important, as ever! A quartz digital watch reading to seconds or even an analogue watch with a second hand will do perfectly. You can calibrate it on the hour using the BBC pips on LW or VHF FM on Radio 4;
the key moment is the beginning of the last [long] pip. Ironically, you can do this also from the clock on your
GPS, which you can also use of course to check your sights when you have done the sums!!
Have Fun!! Practice Makes Perfect!
John Alder
1: For example http://www.ringbell.co.uk/info/hdist.htm
2: For example http://www.rvasclub.org/rvas-news/pdf/cnav.pdf
3: Celestial Navigation for Yachtsmen by Mary Blewitt (ISBN: 9781408132128) [Amazon]
4: Nautical Almanac on line http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.dll
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