Which Way is North? Part 3

The Magnetic Compass
compass   A compass used for navigation is essentially a magnetized needle or bar mounted on a point so that it may freely pivot or swing around in any horizontal direction. In the Northern Hemisphere, the north-seeking end of the needle swings around to align itself parallel with a magnetic line of force, and in doing so, points toward the north magnetic pole. Use the link “Magnetic Effects in Space” in the Internet Resources below to watch astronaut/scientist Owen Garriott on a Skylab mission use a bar magnet to demonstrate the Earth’s magnetic field and lines of force.
   Because the north magnetic pole is not located where the true North Pole (geographic) is located, an adjustment called magnetic declination is made. The magnetic declination is the horizontal angle difference between geographic north and magnetic north. Using the magnetic-field calculator at the NOAA website (see Internet Resources below), you can easily determine the magnetic declination for your location, or any other location on Earth, for that matter. By comparing different locations and dates it becomes apparent that the angle for declination not only varies daily as the north magnetic pole migrates, but there is also a difference in magnetic declination based on both latitude and longitude. For example, at the latitude and longitude of Cambridge Bay, the magnetic declination is 8.07º; using the coordinates for Washington, DC, 39º N, 77º W, the magnetic declination is -10º51’. Negative magnetic declination values indicate that the north magnetic pole is to the west of the location, while positive values have the magnetic pole to the east of the location. From the nation’s capital, the north magnetic pole is toward the northwest, while from Cambridge Bay it would be to the northeast.

   Use this link to see a list of magnetic pole locations from 1590 through 2015 at the NOAA web site.
   Using the years above and this link to see an interactive world map where you may set the date to see the lines of magnetic declination for that time period.

compass   As previously described, a compass has a freely spinning needle balanced on the point from which it spins. When held flat in one’s hand, the needle quickly swings back and forth as it aligns itself with the magnetic lines of force. At lower latitudes, such as those within the continental United States, the compass needle mostly swings left or right as it aligns parallel with the magnetic field and lines of force. As one approaches the magnetic pole, the needle is still parallel with the magnetic lines of force and responding by swinging left or right. However, near the poles, the angle of the magnetic lines of force, with respect to the Earth’s surface, are now approaching vertical; as a result, the compass needle is pulled downward, rather than moving side to side, as would be the case at lower latitudes away from the magnetic pole. In effect, the downward pull on the needle creates increasing resistance on the formerly freely swinging horizontal motion the needle had at lower latitudes. At some point, the needle simply stops moving from side to side.

(Adapted from my January 2013 Scope on the Skies column)

Previous: Which Way is North – Part 1?
Previous: Which Way is North – Part 2?

Internet Resources
Magnetic effects in space—http://archive.org/details/skylab_magnetic_effects
Magnetic field calculators—www.ngdc.noaa.gov/geomag- web/#declination
The Magnetic Sun—http://solar.physics.montana.edu/ypop/Spotlight/Magnetic
Motion of the magnetic pole—http://image.gsfc.nasa.gov/poetry/activity/s8.htm
Planetary magnetic fields PowerPoint—http://education.gsfc.nasa.gov/nycri/units/pmarchase/3b-magnetic_field.ppt


Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

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