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The Earth is an obloid spheroid or much the same shape as an orange round and slightly flattened at the top and bottom For the purposes of elementary navigation, however we can afford to be simplistic and consider the earth to be a perfect sphere

Position on the Earth’s surface is defined in the terms of angular measurement divided vertically or horizontally into 360° degrees

1° (one degree) is equal to 60′ (minutes)

1′ (one minute) is equal to 60″ (seconds)

Degrees are always qualified with the symbol

Minutes and seconds in elementary chart work relate to angular measurement and distance and not to time

The universal unit of measurement at sea is the Nautical Mile This is not the same length as a Statute or Imperial mile The international accepted standard is that 1 minute of arc = 1 nautical mile =1852 meters, equivalent to 60766 feet for the Old Salts The nautical mile is sub divided into tenths, which are known as Cables Cables is considered an old-fashioned term these day with a Nautical mile being expressed as a number and decimal ie 325 Nm However, its usage is still common

The unit of speed used at sea is the Knot One Knot is equivalent to traveling one nautical mile in one hour If we travelled 14 nautical miles in a period of one hour the then our speed would be 14 knots per hour

The Earth’s Axis is an imaginary line through the earth from top to bottom around which the Earth rotates

The Geographical Poles or North and South Poles are the geographical positions on the Earth’s Surface through which the Earths Axis passes

Terrestrial Circles are imaginary circles drawn on the surface of the earth that aid us in navigation Parallels of Latitude and Longitude are Terrestrial Circles so is the Equator

Great Circles are Terrestrial Circles and are circles whose plane passes through the centre of the Earth effectively dividing it equally in two

Small Circles are Terrestrial Circles whose plane does not pass through the centre of Earth

Is a Great Circle whose plane is perpendicular to that the Earth’s Axis midway between the Poles It separates the Earth laterally into two halves The North Hemisphere and the South Hemisphere (Hemisphere meaning half a sphere)

The Greenwich Meridian or Prime meridian is the most significant of all the Meridians of Longitude It is part of a great circle and cuts the equator and all other Parallels of Latitude at right angles It passes through the Greenwich Observatory in London and is named 0 (Zero) degrees Longitude It effectively divides the earth into the East and West Hemispheres and we count our meridians of longitude 0 – 180 degrees East to the right and 0 - 180 degrees West to the left of this Meridian

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