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Terrestrial Navigation

Rotation of the Earth

Rotation of the Earth

The sun rises in the East and sets in the West.

Until the Middle Ages when the earth was thought to be the centre of the solar system, it was accepted that the sun moved in the sky.

However we know better now, yet it still does not change the fact that the sun still rises in the East and sets in the West.

So, with the sun is stationary and the Earth revolving around the sun and also rotating around its own axis, it becomes obvious that the Earth has to rotate on its axis from West to East.

Thus if two points A and B on the surface of the Earth, separated by thousand miles, with B being West of A, are to rotate in the West to East direction with the Earth, then the Sun would first be seen by A and after a certain duration of time B would see the sun.

The directions on the Earth’s surface are NESW. North is taken (excluding the physical North) to be in the direction where the compass points and towards the Pole star (in the Northern Hemisphere).

The opposite of the North Pole is the South Pole.

And the direction of the sunrise is generally taken as East and the direction of the sun set is generally taken as West.

However as explained in later lessons the sun does not exactly rise in the East (090°) in all places of Earth and neither does it set at 270°. The rising and setting direction depending on the latitude among other factors.

The gyrocompass always points to the physical North, since it is not affected by the Earth’s magnetism. Thus a ships head on a gyro compass would always show the correct physical direction – provided the gyro compass has no error.

Unlike a magnetic compass the gyro compass error are not termed East or West. Since the error is a solitary figure for all practical purpose the error is termed ‘High’ or ‘Low’.

The ‘High’ corresponds to a similar Westerly error of the magnetic compass and the ‘Low’ corresponds to a Easterly error of the magnetic compass.

The error correction is also similar and the quote:

‘Error East, Compass Least’

‘Error West, Compass Best’ is changed to

maybe used with the gyro compass, where the words that are substituted are ‘East’ to ‘Low’ and ‘West’ to ‘High’.

Magnetic Course:

The angle between the magnetic meridian and the direction of the ship’s head. It defines the direction of the ship’s head relative to “Magnetic North”. The difference between the two is the Variation.

True Course: After allowing for Deviation and Variation to the Magnetic Course/bearing.

Compass Course: The angle between the compass needle and the direction of the ship’s head. It defines the direction of the ship’s head relative to “Compass North”. The compass course is indicated by the position of the ‘lubber’s line’ relative to the compass card. Both deviation and variation are involved in this correction.