Oct 03 - Sailing South
Noon Position : lat 15 32.0 N, long 26 12.4W (Bearing: 212 T, 104Nm from Verdes)
Distance traveled since Immingham : 2756 nautical miles
Air temperature @ noon today : 27 degrees C
Sea temperature @ noon today : 27.6 degrees C
Wind: Direction NE, Force 4
Having left the cold, foggy shores of the British Isles last
week we are currently cruising south into warmer waters. Just
two days ago we passed the shores of Madeira, hazy with Saharan
dust. We are now past the Canaries and cruising towards South
America and we don't expect to see land again until we get to
Maderia in the haze. Photo taken by A. Liddell
Maths Class in the Middle!
Regular readers of the diary will realise that there is usually a 'science bit in the middle'. As this cruise has no science ongoing at present this has had to be modified slightly. Around midday each day I had noticed that the cadets vanished up onto the bridge and returned a while later muttering numbers under their breath, so I went along to see what was going on. I was very surprised to find them doing maths lessons (or so I thought). They are actually learning to do navigational calculations. There are two parts to this. The first is the calculation of the noon day figures (an abbreviated version is shown at the top of the page). To calculate the distance we have travelled from noon one day to the next you need the latitude and longtitude for each day and then you work out the distance between them. Easy, or so I thought. What I had failed to realise is that it is not a straight line, it is a curved line (arc) across the Earths surface. Then I got another surprise, the Earth is not a sphere! Due to the force generated as it spins on its' access, mass is attracted towards the equator, which makes the Earth an elliptical shape. So to calculate the distance you need to work out the distance along an elliptical arc. No wonder it is so tricky! Once the distance for that 24hour period has been worked out, then the total distance from Immingham can be calculated. Another important part of the noon day figures is to estimate how long it will take us to get to Montevideo (our next way-point) and this is calculated for two average speeds, 11.5 knots and 12.5 knots.
The second part is called celestial navigation. Before the days of GPS (Global Positioning System) sailors calculated their longtitude and latitude using the sun, stars and moon. To do this you need to calculate the height of a celestial body above the horizon at a known time using a sextant.
Photo taken by L Handcock.
Here is Andy calculating the angle of the sun above the horizon at celestial noon. (The sextant has very dark filters to protect his eyes against the sun.) Then I had another surprise, did you know that celestial noon ie the time that the sun is exactly overhead in the sky, is not at 12.00 each day? It varies and 12.00 noon Greenwich Meridian Time is actually an average.
Once you have calculated the height of the sun above the horizon then, using some complicated looking trigonometry, you can work out your longtitude (I used the computer for the maths!) Of course, for this to work it is important to know what the exact time is and this is why celestial noon is used. One of the greatest scientific challenges of the 17th century was to produce a timepiece that was accurate in rough weather and at all temperatures and humidities. Nowadays we have accurate clocks and a reading can be taken at any time as long as you alter the calculations to account for the time. Of course, you could always look at the GPS......but where's the fun in that?
On last weeks diary page the cadets and I were introduced as a newcomers to the ship. We also have two new engineers:
Neil; 3rd engineer, trying to run away from the camera,
Next week - King Neptunes' court
So we'll leave you for another week as we steam southwards heading for the equator, where King Neptune will be waiting for those unfortunates who have never yet crossed to the Southern Hemisphere.