How do we navigate at sea?
How do we know where we are on the Earth’s surface when travelling on the oceans?
- Antoine will be taking part in the 2018 Golden Globe Race, which requires all entrants to use only the same type, or similar equipment, and technology that was carried on board Robin Knox-Johnston’s 1968/69 race winning yacht Suhaili.
- So current GPS technology is not allowed and Antoine will be relying on much earlier methods of determining his latitude (how far north or south he is) and his longitude (how far east or west he is). (Don’t worry, competitors are being carefully tracked and do have more modern aids, but only for use in an emergency!).
- Since ancient times, sailors have known how to determine their latitude by using the positions of the stars. Since the 1700s, an instrument called the sextant has enabled them to do this more accurately - here’s how a sextant works and how you can make one.
A sextant in action! One of the most fascinating sea voyages of the 20th Century was the ‘Kontiki’ in 1947. The explorer, Thor Heyerdahl, believed that people from South America could have settled Polynesia and aimed to carry out some ‘practical archaeology’ by sailing a balsa wood boat from Peru westwards.
The sextant can be used to work out latitude, but how do we decide how far east or west we are? This longitude ‘puzzle’ limited long ocean-going journeys for centuries. A Longitude Prize was put up in 1714 for anyone who could radically improve the method of determining longitude. Click this link to learn more about how a method was eventually found to determine longitude accurately.
Have a go yourself at measuring your latitude and longitude with this step-by-step guide from the ‘Rough Science’ team!
Rough seas ahead?
How do different wind speeds affect the sea and why does the sea affect weather on land?
As Antoine’s voyage continues he’ll see an awful lot of rough sea! The unequal heating of different parts of the Earth by the Sun drives the weather. One very visible effect is the flow of warm and cold air around the planet known as winds. Winds then drive the waves that Antoine will experience. To describe wind speeds, in a way that different observers could agree on, the Irish hydrographer Francis Beaufort (later Rear Admiral Sir Francis Beaufort) devised a scale in 1805. This gives a ‘Beaufort number’ to the wind speed. You may be familiar with the idea of ‘Gale Force 9’ from the daily shipping forecasts.
And here is what the different numbers look like in real life at sea taken by the Open University’s Dr. Mark Brandon:
Wind and weather at sea are part of the current global weather patterns leading to the longer-term idea of ‘climate’ - the weather conditions prevailing in an area in general or over a long period. Here’s an example of how the sea and land interact in terms of weather and climate:
How fast will they be travelling?
Modern day yacht racing compared to the 1960s.
You might assume that today’s yachts would automatically be quicker than those in the 1960s. However, these 2018 boats will race in the absence of modern technology, and the boats taking part are of the same broad type as the boats that sailed the first race (e.g. Knox-Johnston's Suhaili), so there is an argument that the overall speeds of the boats will not be materially faster or slower this time than last time.
However, the current competitors have access to the bank of experience built up by sailors who have navigated these waters since the original race, and so there is more information on prevailing weather patterns, the sea state in different waters, as well as more detailed knowledge of ocean currents. All this information feeds into decision making about the setting of sails and choosing a course, as well as the course made good on a particular leg.
With this in mind, there is an argument that this knowledge will allow the sailors to make better navigational decisions this time around, and so trim off a bit of time compared to the first time.
From a meteorological perspective, however, there could be an argument that says the two years are very unlikely to produce precisely the same conditions. With that in mind, and given the length of the race, the overall time taken to complete the course (in this case the whole world) will simply be minimized by the largest (i.e. the fastest) boat that encounters weather that allows it to sail at its maximum speed, for the longest time, along the shortest (in terms of time) course.
Race organisers currently expect a 260 day trip instead of the 312 days by Robin Knox-Johnston – watch this space!
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