Navigation ( Part 2)

 

                   Navigation ( Part 2)

 

GMT – Greenwich Mean Time

When the sun is on the Greenwich meridian GHA = 0 degree . but the time is said to be 1200 hours GMT . so GMT is ahead of GHA by 12 hours . GMT is the number of hours ,minutes and seconds since the sun crossed the inferior meridian of Greenwich .( The inferior meridian is one which is 180 degree away  ie) on the opposite side of the earth /celestial sphere) .

 

LMT – Local Mean Time

When the sun is on the observer’s meridian LHA = 0 degree but time is said to be 1200 hours LMT . so LMT is ahead of LHA by 1200 hours . LMT is the number of hours , minutes and seconds since the sun crossed the inferior meridian of the observer . ( The inferior meridian is one which is 180 degree away ie) on the opposite side of the earth/ celestial .

Longitude in time = GMT +/- LMT

 

Axis

The axis of the earth is the diameter about which it rotates .

Great Circle

Great Circle is a circle on the surface of the sphere , the plane of which passes through the center of the sphere . There is onlt one great circle through any two point on the sphere’s surface .except if the points are at the two ends of a diameter when an infinite number of great circles are possible.

Small Circle

Small circle is a circle on the surface of a sphere , the plane of which does not pass through the center of the sphere.

Geographic Latitude

Geographic latitude is the angle between the plane of the equator and the vertical at that place. The geographic latitude differs from the geocentric latitude as the earth is not a true sphere. The difference between them is nil at the equator and at the poles they differ by a maximum of about 11.6 minutes at 45 degree N and 45 degree S.

Geocentric latitude = Geographic latitude – ( 11.6 X sin 2 geographic latitude )

 

Nautical Mile

Nautical mile is the length of the arc of the arc of the meridian subtending an angle of  1 minute at the center of curvature of that place . It may also be defined as the length of a meridian between two geographic latitudes which differ by  1 minute , that is  1 minute of d’lat .

Knot

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

 

Geographical Mile

Geographical Mile is the length of the arc of the equator subtending an angle of 1 minute at the center of the earth.

 

Statute Mile

Statute Mile or Land Mile is an arbitrary measure of length equal to 5280 feet .

 

Kilometer

Kilometer is the approximate length of 1/10000 part of a meridian between the equator and the pole .

( 90 degree X 60 = 5400 minutes X 1.8523 = 10,002.43 km )

     

True Course

True course is the angle at the ship between True North and the ship’s head ie) the angle between the true meridian and the ship’s fore and aft line.

 

True Bearing

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

 

Magnetic meridians

Magnetic meridians are lines joining the magnetic poles of the earth . since these poles are not in the same position as the geographic poles, there is an angle between the magnetic and the geographic meridians . The angle between them is known as Variation.

 

Variation

Variation is different at different places it is termed east ,if the magnetic North lies to the east or right of the true North and west if the magnetic North  lies to the west or left of the true North. 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 . The change is called the secular change in variation .

The variation and the amount of yearly change in it are indicated on the compass roses on the charts. The value of the variation at any place may also be obtained from the variation chart of the world.

 

Deviation

Deviation is the angle between the magnetic meridian and the North – South line of the compass card . Deviation is termed easterly if the compass North lies to the East or right of the magnetic North . and westerly if the compass North lies to the west or left of the magnetic north. The deviation of the compass varies as the ship’s head changes . it should be noted that for the same ship’s head , the deviation remains the same for all bearings , as deviation depends on the ships head and not on the bearings.

 

Compass error

The Compass error is the algebraic sum of the deviation and the variation.

 

Compass Course  040

Deviation              15 (E)         E + , W -

Magnetic               055 (M)

Variation                010 (W)    E + , W -

True Course           045 (T)

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Will revert with Part 3, shortly

Good Luck.