Sky Wonders: When is North not North?

Depends on WHICH North you mean and WHEN you mean it.

If you look closely at some maps, you'll notice that there are THREE linear directions referred to as North:

(image courtesy map-reading.com)

Grid North: The north that is established by using the vertical grid lines on a map. Grid north may be symbolized by the letters GN.
True North: A line from any point on the earth's surface to the geographic North Pole, that point on earth containing the earth's spin axis (in conjuction with the South Pole). Lines of longitude are True North lines. True North is usually represented by a star, since in reality the North Star, Polaris, is very close to True North. Can also be indicated by the letters TN.
Magnetic North: The direction to the North Magnetic Pole, as indicated by the north-seeking needle of a magnetic instrument. Usually indicated by the letter N or MN.

Why are they different?

Grid North
Grid North lines are the vertical lines represented on some maps. These GN lines do not necessarily match the True North lines of longitude because a map is a flat (2-D) representation (or projection) of a curved (3-D) surface. Conversely, lines of longitude will not appear perfectly straight on a 2-D map projection.

(image courtesy John Sharry, petroleum geologist)

The map above shows a sample projection (conic) of North America. Notice that the True North lines (lines of longitude) all converge to a single point, the North Pole, but Grid North direction (parallel with the side of the map) doesn't really converge to a single point. Some map projections are better than others, depending on what region of the earth is to be projected. Read more about map projections here: University of Colorado Dept. of Geology.

True North
Ok, so TN should be reasonably accurate, right? As long TN stays on earth, that is.

True North refers to a line drawn from any point on earth to the North Pole. The North Pole, also called Geodetic North, is one of the two points on earth that contain the earth's spin axis (in conjuction with the South Pole, or Geodetic South).

Early navigation used the celestial cousin of the North Pole, called Astronomical North or Celestial North. Celestial North is that point in the night sky around which all the stars appear to rotate (if you live in the northern hemisphere). So, if you were to stand on the North Pole, Celestial North would be directly above you.

It should be made clear that although True North doesn't move over time, Celestial North does. Why? Because the earth's axis wobbles as it spins in space! This "wobble" is called precession and has a period of about 23000 years.

(images courtesy The Electronic Universe)

Earth's precession is caused by the gravitational pull on the earth mostly by the moon, but also by the sun. Celestial North, also called the North Celestial Pole, is roughly 45 minutes of arc (0.66 degrees) away from Polaris, the North Star, which marks the end of the constellation of the Little Dipper. But Polaris wasn't always the North Star. In 3400 B.C., when the Egyptians were building the Great Pyramid of Giza, the closest bright star to Celestial North was the star Thuban in the constellation Draco. As you can see, time plays an important part in calculating star positions.

Magnetic North
Earth has a magnetic field whose magnetic lines of flux begin at a "north pole" and end at a "south pole", just like a magnet. See the lines of magnetic flux in the image below.

(image courtesy NASA)

The needle in a compass aligns itself with these "lines of flux" and, in effect, indicates the north and south directions. Surprisingly, the earth's magnetic "north pole" is not aligned with the True North Pole! This means that the north pointed to by your compass is not True North! This is because the magnetic field depends on the magnetic polarization of the molten rock in the earth's core, which has unevenly distributed magnetism.

However, for practical purposes this difference between MN and TN (called the "declination") can be assumed to be constant for the small regions on earth covered by most maps. For example, in Texas the Declination is about +6 degrees. This means that if your compass tells you you're traveling at a bearing of 10 degrees, you're really going 16 degrees. But this declination doesn't stay put over time. The molten iron in the core moves as the earth's geologic nature is ever changing. This means that, even more surprisingly, the earth's Magnetic North changes over time!

(images courtesy Geologic Survey of Canada)
The left image above plots the position of Magnetic North from its discovery in 1831 in northern Canada until 2001, according to the Geologic Survey of Canada. In those years the MN has moved over 1100 km (660 mi). That's an average 3 miles per year! Most of that movement has occured in the last 35 years when the movement has increased to about 24 miles per year. The image at right above shows that the predicted position of the MN in the year 2050 to be near Siberia, Russia. Read a CNN article talking about the movement of the MN from Canada into Russia.

Not only does the Magnetic North Pole move from year to year, but every few millenia the polarity of Earth's magnetic field REVERSES ITSELF!!!

(image courtesy USGS Earthquake Hazards Program)

This image illustrates the polarity of the rock on the ocean floor as it emerges from an intercontinental rift. Scientists have found that the ocean floor has strips of rock of opposite magnetic polarity that appear to be about 500,000-700,000 years apart. During a reversal period the earth's magnetic field is weak causing compasses to likely be very inaccurate. This weak magnetic field would also allow much more solar radiation to enter earth's atmosphere, likely causing Solar Aurorae to move beyond polar regions.

Read more about magnetic reversals at these sites:

A Last Word About "Orientation"

North was not always the direction considered to be UP on maps. Prior to the popular use of the magnetic compass and stars for navigation, east was considered up on most European maps. There are many explanations for this eastward facing map, including pointing the map to the rising sun, towards the Orient or towards Jerusalem, considered the Holy Land by Medieval Europe. The word Orient is in fact derived from the Latin adjective 'oriens' derived from the verb 'oriri' meaning 'to rise or come forth' which in the English language came to refer to the location of the rising sun at the equinoxes, i.e. East (reference from Wordwizard.com here). The word orientation in English then came to be known as obtaining ones' direction or bearing based on this East-centric map.

North is not South
Now you're probably all wondering about South? So I don't have to write all of this again, just flip everything upside-down (or back to right-side-up for those of you "down under").