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The meridian that joins us
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Geographic coordinates
2 The figure is a representation of the Earth. PN and PS are the geographical poles (points of intersection of the axis of terrestrial rotation with the Earth's surface) and G is the centre of the Earth.

Maximum circles passing through the two poles are called meridians. The biggest circle perpendicular to the axis of terrestrial G is the equator. The smaller circles parallel to the equator are called parallels.

The position of any point on the earth's surface, is determined by its meridian and its parallel. This is defined as its coordinates: latitude and longitude.

The latitude φ of a point is the angular distance between the Equator and the parallel passing through P. Values φ may vary from range 0° (for a point in Ecuador) to 90° (for the poles) and it is North for points placed up the equator and South for points placed south of the equator.

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 To define the longitude, it is necessary to choose arbitrarily a meridian origin, as Meridian 0º. The ideal is the one of Greenwich, near London, where in the seventeenth century one of the first astronomical observatories in the world was built.

The longitude λ of the point P is the angular distance between the Greenwich meridian and the one passing through P. λ may range from 0° (for a spot right in the Greenwich meridian, as Castellón or Monte Perdido in the Pyrenees) and 180º (for a spot on the opposite side of the Earth, such as the eastern tip of Siberia) and it may be East for place eastern of the Greenwich meridian (Poland), or West for places western of the Greenwich meridian (Portugal)

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The Earth's magnetic field
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Although the Greeks already knew the properties of magnetite and that Chinese sailors used, 2,000 years ago, a primitive compass to guide them, it was not until 1,600 when W. Gilbert, experimenting with areas of magnetite, concluded that the Earth acts as a giant permanent magnet and that the Earth's magnetic field was of internal origin. In 1839, Gauss proved that most of the magnetic field has its origin in the interior of the Earth, although there was a small component of the field of external origin.

So that Earth's magnetic field can divided into the following components:

The transitory or external field, which is created by the interaction of the ionosphere with the solar wind which includes magnetic storms often accompanied by aurorae borealis.

Magnetic anomalies due to the concentration of magnetic minerals and rocks in the earth's crust.

The main field of internal origin or internal field, which equivalent to a dipole located in the interior of the Earth.
What is the magnetic declination?
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The magnetic declination is the angle formed between δ the geographic North pole and the magnetic North pole. It is considered positive when the angle is measured eastwards from the geographic North, and negative when it is to the West of the geographic North.


Due to the dynamic nature of the Earth's magnetic field, the declination is also changing, and for a same place, the decline changes from one given date to another.
Determine the direction of the meridian passing through
the I.E.S. Juan de Garay of Valencia
As the direction pointing the magnetic needle of the compass is towards the magnetic North pole and the direction of the meridian in the place to the geographic North pole (in the direction we want to find out), forming an angle that is just the we have defined as magnetic declination easily we can get the address of the meridian if we know the value of the magnetic declination
The magnetic declination for Valencia in the date 06-01-2008 is 1º 0´ West
We will support the compass on a horizontal surface, and determining the magnetic North pole and knowing the magnetic declination of Valencia in this date, the direction of the geography North is obtained easily.
Geography Meridian = Magnetic Meridian + 1º
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