Following the Antarctic Circumpolar Current
Joséphine's trajectory on 16/12/2008 superimposed on the ocean topography map. The buoy more or less follows topographic contours. The steep ocean gradient visible over a few degrees of latitude is where the Antarctic Circumpolar Current flows.
Joséphine's trajectory is relatively straight; it is very similar to that of the buoys in the case study "A buoy 'moves straight ahead'", e.g. Carioca (2005-2006 and 2006-2007), or Julius (2004-2005 Vendée Globe race). If we superimpose its trajectory on ocean topography maps, the buoy can be seen more or less following the contour lines of the same height (i.e. 'isolines').
Joséphine's trajectory on 24/02/2009 superimposed on maps showing ocean topography, satellite-measured temperature and current models (from top to bottom). The buoy more or less follows the isolines of the first two maps.
In Google Earth (open the file for any given date by clicking on the link "View maps with Google Earth" at the top of the map page; with some web browsers, you will need to download the file to your computer in order to open it with Google Earth):
Look at the time between two successive locations by clicking on two successive points reached by Joséphine and looking at the dates and times in the bubble that opens. Calculate the difference in order to obtain the time taken for the buoy to travel between the two points.
Google Earth also allows you to measure the distance between two points on the globe (‘ruler’ tool). Use this tool to measure the distance between your two points.
The buoy's speed is calculated as follows: speed = distance/time. Do the exercise for several different locations and compare speeds. Make further comparisons with the speed map available on the Argonautica website (note that these maps give speed in metres per second).
Joséphine’s speed: 2.5 km/h