All at Sea

Where Next?
Bob Williams
Sun 15 Jul 2007 19:07

Position:  40 41.0 N  071 50.5 W

Course: East Northeast, Speed: 6 knots

Wind: South 15 knots.


We continue to make good progress in a fair and consistent southerly breeze.  Our 24 hour midday to midday run was 138 nautical miles, that’s an average of 5 ¾ knots, which for Sylph is pretty fair.  For the uninitiated a nautical mile is slightly bigger than a statute mile and a whole lot bigger – by almost twice – than  a kilometer.  Technically a nautical mile is 1 minute of arc (i.e. 1/60th of a degree) subtended from the centre of the earth to the earth’s surface which on average is 6080 feet, so it is fact an angular measurement rather than strictly a distance and can vary slightly depending on where you are because the earth is not a perfect sphere.  So when I swing my sextant, rarely in these days of GPS, the angle I measure can be translated relatively easily to the chart.  A good sextant by the way can measure to an accuracy of 1/10th of one minute of one degree – that’s a pretty small angle.  Traditionally in the world of nautical navigation the day starts and finishes at midday because this is when the navigator would take the meridian passage, i.e. when the sun is directly over your local meridian, which by definition is your local noon.  If you have read the book “Longitude” you will be aware of how long Western civilization struggled with finding a way of determining longitude.  Latitude on the other hand is much simpler, all I need to do is to measure the angle of the sun when it is directly north or south of me (i.e. directly over my local meridian) and if I know the sun’s declination (the angle the sun is away from the equator) then with a few very basic sums I can work out my latitude.  People have kept records of the sun’s declination since before the Egyptians, indeed it seems that the pyramids are very accurately aligned with the seasonal movements of the sun and of course there is Stonehenge, so latitude has never presented the problem that longitude has, solved by a clock maker.


And why a knot I hear you ask?  All right, I’ll spare you my didactic instincts and will leave this one for tomorrow.  All fascinating stuff.


And speaking of the earth not being a perfect sphere, apparently a part of the reason is due to the weight of ice on the continent of Antarctica which actually presses the land mass below sea level.  So I guess if all the ice melted Antarctica would cease to exist, or would it “bob up” to the surface?  Well I am sure things won’t get that bad, will they?


Sylph and crew are well.