Featured Map for 14 March 2023:
Pi(rates) and Rhumb


Today is Pi day: 3/14 in the month/day notation used in the United States. With Circle in this web site's name, Pi Day is worth celebrating!

Pirates are notorious for their love of rum (and for other things), but tended to haunt narrow straits where it was easier to prey on other ships. Therefore, their navigation was mostly tactical. For longer-range navigation on long ocean voyages, sailors used rhumb lines: paths of constant heading. Also known as loxodromes, these paths were usually longer than a geodesic (great circle) path but were easier to navigate in a time when the primary tools were a magnetic compass, a sextant, and maybe a decent chronometer.

On a Mercator projection, rhumb lines are always straight lines, hence the popularity of that projection (which is not supported by the Great Circle Mapper ... yet).

Today's Featured Map introduces rhumb lines as a path type, using the route of the RMS Titanic as an example, using airports as proxies for the seaports used. Titanic sailed from Southampton on 10 April 1912, stopping at Cherbourg, France, before continuing overnight to Cork Harbour, Ireland (in what was then Queenstown, renamed Cobh in 1920). From there, it began its Atlantic crossing to New York. At 11:40 pm on 14 April 1912 it hit an iceberg about 1,250 miles (2,012 km) east of New York. The map illustrates Titanic's path as white rhumb lines; the red line is to geodesic ("great circle") path for the Atlantic crossing.

Rhumb lines have been available in the Great Circle Mapper for years but not exposed until now. They may not see much use as more than a curiosity, though there is precedent for oddball features being popular. Please share any interesting maps you create using rhumb lines!

Pirate flag aka Jolly Roger

Oh, you wanted pirates! As noted above, pirates probably didn't have much need for rhumb lines, but Queen Anne's Revenge, the flagship of infamous pirate Blackbeard, started life as a French slave ship named La Concorde. It's last voyage in that role was across the Atlantic from Benin, in West Africa, to its capture in the Caribbean near Martinique. That voyage would have looked something like this, with the rhumb line in green and the geodesic path beyond West Africa in red:

Map of Bénin to Martinique

References and additional information:


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