Saturday, February 26, 2011

Sea Turtle mythology (navigation)

An article in "Wired" claims that sea turtles navigate by sensing the earth's magnetic field (quite possible) and can sense longitude magnetically (not likely at all). The author is clearly unfamiliar with magnetic compasses, magnetic variation, and magnetic dip, all subjects well known to seamen for hundreds of years. Nor has he read the classic investigation of homeing pigeon navigation done many years ago.
Finding longitude at sea has been intensively studied for centuries. The only way to do it magnetically is to measure magnetic variation. The magnetic poles are not exactly at the geographic poles which means magnetic north varies from true north. Variation ranges from zero to maybe 20 degrees, depending upon where you are. Variation is measured by comparing magnetic north (compass reading) with true north. For human navigators true north is found by observing Polaris, the north star, or by use of a gyro compass (invented around the time of WWI). It is doubtful that sea turtles can use either method.
The earth's magnetic field lines run horizontally at the equator and run nearly vertically right at the magnetic poles. The angle of the magnetic field with respect to the ground is known as magnetic dip, and can be measured with a simple apparatus, essentially a magnetic compass mounted on its side. Human navigators do not use magnetic dip to find latitude because measuring the height of the sun at noon gives a much more accurate latitude indication than magnetic dip does. But the magnetic dip method could be used by sea turtles.
I doubt that sea turtles are better animal navigators than homing pigeons are. Homing pigeons are so good at finding their way back to their homes that they were used to transmit messages up until the invention of portable two way radios in WWII. Pigeons navigate partly by observing the sun and partly by sensing the earth's magnetic field. On overcast days pigeons depend upon magnetic navigation. Attaching small permanent magnets to the pigeon's feet will disable their magnetic sensing. Pigeons released with magnets on overcast days always get lost.
I suspect the real truth of the sea turtle story is that the turtles can sense the earth's magnetic field and can navigate about as well as homing pigeons, using the same techniques. I do not think the turtles, or the homing pigeons can sense longitude.

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