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### SHAPE OF THE EARTH :

The shape of the earth is called an Oblate spheroid. The equatorial diameter is more than the polar diameter.

**Equitorial radius = 6378.16 Km**

**Polar radius = 6356.77 Km**

As the difference is negotiable the navigators consider it as a perfect sphere.

# Definitions

**Axis**– The axis of the Earth is the diameter about which it rotates.

**Poles**– The geographical poles of the earth are the two points where the axis meets the earth’s surface.**Equator**– The Equator is a great circle on the surface of the Earth, the plane of which is perpendicular to the Earth’s axis. The Equator divides the Earth into the north and the south hemispheres.

**Great Circle**– It is the circle on the surface of the sphere, the plane of which passes through the center of the sphere.

**Small Circle**– It is the circle on the of a sphere, the plane of which does not pass through the center of the sphere.

**Parallels of latitude**– Parallel of latitudes are small circles on the surface of the Earth’s surface, the planes of which are parallel to the plane of Equator. All parallels run East-West.

**Meridians**– They are semi-great circles on the Earth, joining the two poles.

**Prime Meridian**– is the meridian which passes through Greenwich. The other meridians are named East or West from Prime meridian.**Difference in Latitude**– The d’lat between two places is the arc of meridian or angle at the center of Earth contained between the parallel of latitude through the two places.**Longitude of a place**– is the arc of the Equator or the angle at the poles contained between the Prime meridian and the meridian through that place.**Difference in Longitude**– The d’long between two places is the shorter arc of the Equator or the smaller angle at the poles contained between the meridians through the two places.**Mean Latitude**– The mean latitude between two latitudes is the arithmetic mean between them.**Nautical mile**– The nautical mile at any place is the length of the arc of a meridian subtending an angle of 1’ at the center of curvature of that place. The arc subtended by the same angle of 1’ at the pole is larger as compared to the Equator. Therefore we use a standard value for the nautical mile.**1 nautical mile = 1.852 Km**

**Knot**– is a unit of speed equal to one nautical mile per hour.

**Geographical mile**– is the length of the arc of the equator subtending an angle of 1’ at the center of the earth.

**Statute mile**– or land mile is an arbitrary measure of length equal to 5280ft.

**True north**– The north direction of a meridian is true north.

**True course**– is the angle at the ship between True north and the ship’s head, that is, the angle between the true meridian and the ship’s fore and aft line. It is measured clockwise from true north and is expresses in three digits.

**Ship’s head**– Direction of head of ship at any point of time is ship’s head.

**True bearing**– The true bearing of an object is the angle at the observer between True North indicated by the meridian and the line joining the observer and the object.

**Variation**– is the angle between true meridian and magnetic meridian and is measured east or west from true north. It is purely due to Earth’s magnetism. It varies from place to place. The value of the variation at a place is not constant. It changes because the position of the magnetic poles of the Earth is constantly changing. This change is called the secular change in variation.**Deviation**– is the angle between the magnetic north and the compass north and is measured east or west from the magnetic north. The deviation is because of the ship’s magnetism and it depends on the ship’s head.**Compass error**– The compass error is the algebraic sum of the deviation and the variation.

Article by Ritika Singh(Tolani Maritime(2020-2022),Sailing as Deck Cadet now as in 2022)