For Berrimilla's first circumnavigation, the International Space Station
and the North West Passage, go to

Saturday, November 21, 2009

On ignoring the oblate shape of our spheroid

The GPS has two sets of numbers upon which we gaze with hopeful yearning - our latitude and longitude co-ordinates. Each reads degrees and decimal minutes & right now our latitude is 26.07.897 S read as twenty six degrees seven point eight nine seven minutes south. For the navigationally challenged, one minute of latitude is equal to one nautical mile so we are roughly 7.9 miles south of 26 degrees. The last digit (the 7 of .897) is one thousandth of a nautical mile or about 2 metres so as we sail along, the numbers increase or decrease depending on our direction. We are heading south west, so the latitude numbers (degrees and minutes south) are increasing by roughly 0.005 for every boat length we sail. The longitude numbers (now 012.38.023 W) are decreasing by roughly the same amount. Only roughly because a degree of longitude at 26 south is less that a nautical mile. Purists, please ignore the problems raised by diagonals and oblate spheroids for this little exposition. For the first time for what seems days, both sets of numbers are counting in their respectively correct directions. South is increasing and west is decreasing. YAY!

Another concept is Velocity Made Good or VMG. This is a calculated number based on our course and speed over the ground relative to where we are going. It is almost always different from our speed through the water and our speed over the ground but it is the best indicator of how efficiently we are sailing the boat and choosing our courses. On the last tack, our VMG for Cape Town was about 1 knot - then came the wind change we have been crossing all the appendages for and we tacked and now VMG for CT is 4.2 knots - much better but still not good enough to get us there by Dec 5th. We need a constant VMG of about 4.8 for that. Our speed through the water is about 6.2 knots and speed over the ground is 5.4 knots so we are in an adverse current of about 0.8 knots. Our required course over the ground for CT is 130M and we are actually making 167M which explains some of the discrepancies. Perhaps another burst on the difference between Magnetic course (M) and True course (T) and variation in another post.

Deborah, thanks for ISS and Atlantis info. As you can imagine, a big news hole out here - can't even get the Beeb world service without major hassle. I'm about to try using the mast as an antenna. Big Hi to Andrew - the Needles in 90+ knots I can only imagine.

Another word of explanation: Our ISS viewing times are roughly the 20 minute periods before sunrise and after sunset - while the ISS can see the sun and reflect its light towards us if they are anywhere near us bur as the sun is below our horizon the sky is dark enough for us to see the reflection quite easily.

Norm - thanks - jeers and ribald laughter from the crowd is what gets one foot out in front of the other again and again in those last 6km.