Units of distance at sea

UNITS OF DISTANCE at SEA Statute & Geographical Miles • A statute mile is 1625 metres (5280feet) and has no relevance to us as it is generally related to land based distance • A geographical mile is the length of an arc at the equator, subtended by one minute at the earth’s centre and is equivalent to 1855.4 metres (6087.2feet) Units of Distance at Sea • As we all know the earth is not a perfect sphere • In fact, the earth has a squashed spheroid shape that is closer to an oval rather than spherical in shape • This means that the curvature of the earth at the Poles is less sharp than that at the equator The Sea Mile • At any place on the earth’s surface the sea mile is the length of the meridian which is subtended by an angle of 1’ at the centre of curvature at that place mile Diagram is exaggerated for the purpose of demonstration Np 1’ Equator Centre of Curvature The Sea Mile • A sea mile varies between 1842.8 metres at the equator to 1861.7 metres at the Poles • This variation is a result of the difference in earth curvature at the mile Equator & the Poles Np 1’ Equator Centre of Curvature The Sea Mile • As a result the centre of curvature will be further away from the surface at the Poles than at the equator • So! An angle of 1’ at the Polar centre will give a greater arc at the mile surface than at the Equator Np 1’ Equator 1’ Centre of Curvature Centre of Curvature mile Units of Distance • A Nautical Mile is fixed by International agreement at 1852 metres • A Cable is taken as 1/10th of a Nautical Mile • A Knot is the maritime unit of speed and is ‘one Nautical Mile per hour’ Rotational Speed of the Earth If the earth is rotating at a constant speed, the speed of rotation of the Earth’s Surface at the Equator is faster than at positions approaching the Poles The speed of rotation of the earth’s surface at the Poles is ZERO Therefore, the speed of rotaton of the earth’s surface is a function of Latitude Rotational Speed of the Earth At the Equator the earth rotates @ 150 per hour Which is the equivalent of 900 miles per hour While at the Poles the speed is ZERO The Latitude of the Equator is 00, the Cosine of which is The Latitude at the Poles is 900, the Cosine of which is 900 x 0.5 = 450 miles/hour The speed of the surface of the earth is therefore the speed at the Equator multiplied by the Cosine of the Latitude e.g In Lat 600 the speed of rotation is 900 x Cos 600 600 N