The Earths Feature

Equatorial radius > Polar radius

Equatorial circumference > polar by about 72 km

Avg radius 6400km

Avg circm 40000km

Earth rotates west to east

Parallels of Latitude – Parallel of latitudes are imaginary small circles on earth surface that are parallel to the equator. They are named N or S according to its hemisphere.

Geocentric Latitude- Latitude of a place is a arc of meridian , Or the angle at the center of earth , Measured between equator and parallel of latitude through that place. (named N or S according to hemisphere).

Geographic Latitude – Is the angle between the plane of equator and Vertical at the place .

Geographic and geocentric latitude vary as the earth is not spherical . This variation is maximum at 45 degree North and 45 Degrees south and the Difference is 11.6′ .

1 Knot is a unit of speed equal to one Nautical mile per hour.

Geographical Mile – Is the angle of the arc of the equator subtending angle of 1′ at the center of earth . It is constant in length = 1855.3m .

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

Cable – 1/10th pf a nautical mile.

True course – Is the angle at the ship between True north and ships 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.

Compass Bearing – Angle at the Observer between compass north and the line of sight of object.

Relative Bearing – Angle at the Observer between Ships head and the line of sight of object.

Deviation – Is the angle between the magnetic meridian and North-South line of compass card. Deviation is termed easterly if the compass north lies to the east or right of magnetic north and westerly if the compass north lies to the west or left of magnetic north. The deviation of a compass varies as the direction of the ships head changes.

Variation – Since Magnetic poles and Geographical poles are not in the same position, there is angle between the magnetic meridian and geographic meridian. The angle between them is known as Variation.

The variation at a place is not constant .It changes because the magnetic poles of the earth is constantly changing.