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
1 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
1 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
1 mile

surface than at the Equator
Np

1’
Equator
1’
Centre of Curvature

Centre of Curvature

1 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 1
The Latitude at the Poles is 900, the Cosine
of which is 0


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