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),
including:
- 4. the north (X) component of H
and
- 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.
http://www.geolab.nrcan.gc.ca/geomag/cgrf_e.shtml
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 ?...
Gord
Linear Mode
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