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Our Position on the Earth’s Surface:
When we are dealing with elementary navigation we can assume that the world is perfectly spherical, however in actual fact it is an oblate spheroid being slightly flattened at both poles not unlike an orange This is because the Earth is not truly a solid and because it has a liquid core the centrifugal force of the Earth's rotation causes it to bulge around the equator It is also tilted on its axis at an angle of 235 degrees but for our purpose we will consider it to be upright
Note: Although traditionally arc was always expressed as degrees minutes and seconds due to the use of digital displays on navigation equipment today it has become more commonplace to express seconds as a decimal of a minute ie 24°163'N rather than 24°1618"N
Our position at any time and at any place on the earth’s surface is defined in terms of angular measurement relative to the earth’s centre That means that positions are expressed in degrees minutes and seconds of Latitude or Longitude It is important therefore before we attempt Elementary Chartwork that we understand the basic terms and definitions
The Earth revolves around a central axis that runs through its centre from the North Pole to the South Pole if it was possible to view the Earth from some point in space directly above the North Pole we would observe that the Earth rotation is in anticlockwise direction moving from the West to the East The time that it takes to complete one complete revolution is one day
When we refer to our position on the earth’s sphere in Latitude and Longitude we are referring to imaginary circles that are drawn on the earth’s surface These terrestrial circles fall into just two groups and they are Great Circles and Small Circles Note that although there are 360 degrees in a circle and we use 360 degrees when finding our direction by compass our positions are expressed as zero degrees to 180 degrees east or west of Greenwich or zero to 90 degrees North or South of the Equator
Positions in chartwork are expressed Latitude first North or South followed by the Longitude East or West, however in the following paragraphs I will cover Meridians of Longitude first because its important to understand the Great Circle Note there are many marine disciplines that use Easting’s and Northing’s and UTM particularly in survey These are beyond the scope of this publication you just need to be aware they exist so you don’t get caught offside
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