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 statute mile is 1625 metres 5280feet

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 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)

• 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

• 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

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

• 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

1 mile

Diagram is

exaggerated for

the purpose of

demonstration

Np

Centre of Curvature

1’

Equator

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 Equator & the Poles

• 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 Equator & the Poles 1 mile

Np

Centre of Curvature

1’

Equator

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 surface than at the Equator

• 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 surface than at the Equator 1 mile

Np

Centre of Curvature Centre of Curvature

1’

1’

1 mile Equator

Units of Distance

• A Nautical Mile is fixed by International

agreement at 1852 metres

• A Cable is taken as 1/10th of a Nautical

• A Nautical Mile is fixed by International

agreement at 1852 metres

• A Cable is taken as 1/10th of a Nautical

• 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’

• 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

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

While at the Poles the speed is ZERO

The Latitude of the Equator is 00, the

Cosine of which is 1

The Latitude at the Poles is 900, the Cosine

of which is 0

The speed of the

surface of the earth

is therefore the

speed at the

Equator multiplied

by the Cosine of

the Latitude

900 x 0.5 = 450 miles/hour

60 0 N

the Latitude

speed of rotation is