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Old 16-05-2004, 09:40   #1
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Where is North ?

So - Where is North ?

This year (2004), Magnetic North is at 82.3 Degrees Lat. North x 113.4 Degrees Lon. West - while True (Geographic) North is at 90 Degrees Lat. North x 0 Degrees Lon.
The longitude at the Pole is whatever you choose, as all lines of longitude converge at the poles.

The North Magnetic Pole is distinct from the North Terrestrial Pole (True or Geographic N), the fixed point that marks the axis of the turning planet. The magnetic pole is currently about 966 kilometers (600 miles) from the geographic one.

As long ago as the 15th century, mariners noticed that the needle of a magnetic compass does not point accurately to Earth's true north. Columbus, for instance was aware of this on his voyages across the Atlantic in the 1490s. Actually, the needle makes an angle with true north, and that angle varies from place to place on the Earth's surface. This means that there is a different magnetic variation for different places on Earth. These variations were investigated on a famous 17th century voyage by the great scientist and astronomer Edmund Halley.

The compass changed little in design between around 1550 and 1750. The azimuth compass was a step up from the standard mariners compass. These compasses incorporated a means of aligning the compass with a celestial body such as the Sun or the Pole star. The reading from this alignment would then give another reading for north which could be compared with that given by the compass needle thus allowing the variation to be easily read.

The Earth's magnetic field is shaped approximately like that of a bar magnet and, like a magnet, it has two magnetic poles, one in the Canadian arctic, referred to as the North Magnetic Pole, and one off the coast of Antarctica, south of Australia, referred to as the South Magnetic Pole. At the North Magnetic Pole the Earth's magnetic field is directed vertically downward relative to the Earth's surface. Consequently, magnetic dip, or inclination is 90̊ . In addition, the North Magnetic Pole is the eventual destination for a traveller who follows his or her compass needle from anywhere on Earth.

The North Magnetic Pole is slowly drifting across the Canadian Arctic. The Geological Survey of Canada keeps track of this motion by periodically carrying out magnetic surveys to redetermine the Pole's location. The most recent survey, completed in May, 2001, determined an updated position for the Pole and established that it is moving approximately northwest at 40 km per year. The observed position for 2001and estimated positions for 2004 are:
2001 - 81.3 Degrees Latitude ( ̊N) x 110.8 Degrees Longitude ( ̊W)
2004 - 82.3 Lat. N x 113.4 Lon. W.

The Earth's magnetic field, as measured by a magnetic sensor above the Earth's surface, is a composite of several magnetic fields generated by a variety of sources. These fields are superimposed on each other and through inductive processes interact with each other. The most important of these sources are (1) the Earth's conductive, fluid core; (2) the Earth's crust and upper mantle; (3) the ionosphere; and (4) the magnetosphere.

The Earth's outer core generates more than 95 percent of the geomagnetic field. This portion of the geomagnetic field is represented by the 2000 International Geomagnetic Reference Field (IGRF) charts. The IGRF model and its secular variation (annual change) consist of a spherical harmonic equation of degree and order 10. This equation is based on several proposed geomagnetic models, which are weighted according to their judged validity. The IGRF model and its secular variation are updated every five years. Each model is valid from its base year through the next five years.

The IGRF and other geomagnetic models are used for several navigational and global positioning applications including air and sea navigation, satellite positioning, and GPS readers and recorders. It is also used for geophysical investigations of the Earth's crust, mantle, core, ionosphere, magnetosphere, and magnetic anomalies.

The IGRF charts are a series of five maps that depict the inclination, declination, horizontal intensity, vertical intensity, and total intensity of the Earth's magnetic field.

To measure the Earth's magnetism in any place, we must measure the direction and intensity of the field. The Earth's magnetic field is described by seven parameters. These are
1. declination (D),
2. inclination (I),
3. horizontal intensity (H),
- 4. the north (X) component of H
- 5. east (Y) components of H
6. vertical intensity (Z),
7. total intensity (F)

The parameters describing the direction of the magnetic field are declination (D) and inclination (I). D and I are measured in units of degrees, positive east for D and positive down for I.

Magnetic declination is the angle between magnetic north and true north. D is considered positive when the angle measured is east of true north and negative when west.

Magnetic inclination is the angle between the horizontal plane and the total field vector, measured positive into Earth.

The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. You will often hear the terms "variation", "magnetic variation", or "compass variation" used in place of magnetic declination, especially by mariners.

Unfortunately, the annual change corrections (Secular variation) given on most of maps & charts cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner. If accurate declination values are needed, and if recent editions of the charts are not available, up-to-date values for Canada may be obtained from the most recent geomagnetic reference field models produced by the Geological Survey of Canada.

The deflection of the needle of a magnetic compass due to masses of magnetic metal within a ship on which the compass is located is called deviation. This deflection varies with different headings of the ship. The deviation is called easterly and marked plus (+) if the deflection is to the right of magnetic north, and is called westerly and marked minus (-) if it is to the left of magnetic north. A deviation table is a tabular arrangement showing the amount of deviation for different headings of the ship. Each compass requires a separate deviation table.

From the Stars:

Astronomers usually call the Little Dipper (Little Bear to some) Ursa Minor (Latin for little bear). By far the most important and famous star in Ursa Minor is the North or Pole Star, Polaris.

This is the star at the very end the Little Dipper’s handle (or of the little bear’s long tail). Another way to find find the Pole Star, Polaris, is by following a line drawn through Dubhe and Merak, the two end stars in the bowl part of Ursa Major (Big Dipper / Great Bear).

The reason Polaris is so important is that it is almost directly above the North Pole. This means you can use it like a compass to find north. Also, the angle of the star above the horizon gives you your 'latitude’ (north-south position on the Earth's surface). For years, sailors relied on the Pole Star for navigating at sea, with the help of instruments like quadrants and astrolabes.

The ancient Greeks realized that Polaris did not mark the pole exactly. We now know that the earth’s axis moves slowly backwards and forwards over thousands of years, so the star nearest the pole changes over time. About 5000 years ago a star called Thuban was the Pole Star. In about 5000 years’ time, a star called Alderamin in the constellation Cepheus will be nearest the pole. Eventually, in about 28,000 years, Polaris will be the Pole Star again – for a time.

So - Where is South ?...


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Old 16-05-2004, 13:26   #2
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Then throw this on top of that to totally mess things up...

"Science leaves no doubt about the possibility of sudden reversals of the Earth’s magnetic poles. They have happened many times in the past and will happen again. The current polarity of the Earth’s magnetic field can be understood by visualizing a gigantic magnetic bar stuck through the top and bottom of the planet. The magnetic field emerges from the south magnetic pole located in the Southern Hemisphere and returns to the north magnetic pole located in the Northern Hemisphere. The actual axis of the magnetic poles is tilted about 20 degrees from the axis of the Earth’s spin. The magnetic poles constantly “drift” in their physical location, and, in recent years have drifted with more frequency. Sources: National Geophysical Data Center; Astronomy, by Michael Zelik (1985)

During the past 3.5 million years, the magnetic poles of the Earth have shifted at least nine times. This has been determined through sampling of the “magnetic records” formed by rock in the ocean beds and in ancient lava formations. It is not known how and why the magnetic poles can reverse, nor is it exactly known what the effect on life would be. Scientists believe that the Earth’s poles reverse an average of every 200,000 years, but the time between reversals has varied widely. The Sun reverses its magnetic poles fairly routinely: essentially every 11 years. Sources: National Geophysical Data Center; Astronomy, by Michael Zelik (1985)"

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Old 17-05-2004, 03:52   #3
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Where is South?

Observed position of the South Magnetic Pole
(2001) 64.7̊ S 138.0̊ E
Source: Canadian Geologic Survey

South Magnetic Pole Movement:
Source: NOAA (NGDC)

Another pole position is the geomagnetic dipole (geocentric dipole). This is the pole position based on the first three terms of the current International Geomagnetic Reference Field, a model of the Earth's main magnetic field. Using the IGRF, and computing the symmetric positions where the dipole would intersect the Earth's surface, the pole positions are:
North Pole: 79.3N, 71.5W
South Pole: 79.3S, 108.5E
These positions are frequently used to generate the geomagnetic coordinate system.

Images from the World Magnetic Model:

What happens to my compass at the magnetic pole?
A magnetic compass needle tries to align itself with the magnetic field lines. However, at (and near) the magnetic poles, the fields of force are vertically converging on the region (the inclination (I) is near 90 degrees and the horizontal intensity (H) is weak). The strength and direction tend to "tilt" the compass needle up or down into the Earth. This causes the needle to "point" in the direction where the compass is tilted regardless of the compass direction, rendering the compass useless.

What happens to my compass in the southern hemisphere?
For a compass to work properly, the compass needle must be free to rotate and align with the magnetic field. The difference between compasses designed to work in the northern and southern hemispheres is simply the location of the "balance", a weight placed on the needle to ensure it remains in a horizontal plane and hence free to rotate. In the northern hemisphere, the magnetic field dips down into the Earth so the compass needle has a weight on the south end of the needle to keep the needle in the horizontal plane. In the southern hemisphere, the weight needs to be on the north end of the needle. If you did not change the weight, the needle would not rotate freely, and hence would not work properly.

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Old 17-05-2004, 20:17   #4
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For Canadians South is when and where the butter melts.
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Old 18-05-2004, 13:38   #5
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Of course, the problem of the needle wanting to point to it's toes or stand on it's head is not going to be a major problem, as it's rather hard to actually get your boat to the actuall poles. One problem that is often encountered though, is magnetic variances due to bedrock type and formation below and magnetic interference from submarine cables. We have both instances around NZ waters, that can make Autopilots taking dramatic changes in course.

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