In the past month or so I have seen several news articles about the Earth’s magnetic field, which got me interested in modeling and analyzing it. It started with reports that some airports in Florida were renumbering some of their runways to account for changes in the magnetic field. One of the latest summary articles can be found here. Most runways in the U.S. are identified by a number from 1 to 36, indicating the approximate magnetic heading (in degrees, divided by 10) on which airplanes take off or land. For example, a north-south runway would be marked 18 at the north end for takeoffs and landings to the south, and marked 36 at the south end for takeoffs and landings to the north. As the magnetic declination in a region changes over time, so does a runway’s magnetic heading.
Despite some amusing religious speculation about this being a sign of the end of days (see here), this is nothing new. Wikipedia has a convenient animated GIF showing the changes in magnetic declination over the last 400 years or so.
The Sun-Sentinel article linked above describes these changes in a rather interesting way, stating that “magnetic north, the point at the top of the Earth that determines compass headings, is shifting its position at a rate of about 40 miles per year.” This seems like a slightly misleading way to characterize the effect on compass headings. The absolute position of the pole does not really mean much to a person trying to navigate with a compass. I think a more useful metric is the change in magnetic declination over time. For example, the following plot shows how much your compass heading changed between 2000 and 2010:
The changes in most of the U.S. are relatively minor, on the order of one degree or so, although you can see the more extreme behavior in Canada nearer the magnetic north pole.
The second interesting bit of news from just last week involved scientists studying how loggerhead turtles use their sense of the Earth’s magnetic field for navigation… but in a slightly more interesting way than you might think. A good detailed Discovery blog post is here.
The idea is rather simple to state: in addition to using magnetic declination to provide a sense of direction, loggerhead turtles seem to use magnetic inclination together with overall field intensity to sense their location as well. (Inclination is the angle between the magnetic field vector and the local level plane tangent to the Earth ellipsoid.) That is, the turtles “map” magnetic inclination and field intensity into latitude and longitude to figure out where they are, then use magnetic declination as a “compass” in the usual way to find the desired direction to swim to get there.
The blog post makes the interesting comparison with the “longitude problem” of marine navigation in the 18th century, solved with robust and accurate timekeeping. How much information does the magnetic field vector– not just its declination, but its precise direction including inclination, and overall magnitude– provide about your position, at least within certain regions of the Earth’s surface?
Following is a plot showing magnetic inclination over the Earth. The two reference points mentioned in the turtle study were at approximately the same latitude, one near Puerto Rico, where inclination is approximately 44.5 degrees, and the other near the Cape Verde Islands off the coast of Africa, where inclination is only 14 degrees.
- Langel, R. A. “The Main Field.” Rpt. In Geomagnetism Vol. 1. Ed. J. A. Jacobs. London: Academic Press, 1987. 249-512.