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Appendix B International System of Units
In SI system, the fundamental units are:
• length - metre (m),
• time - second (s),
• mass - kilogram (kg),
• temperature - kelvin (K).
Prefixes are attached to names or symbols of SI units to form powers-of-ten
multiples. Table B.1 lists the prefixes and associated SI symbols likely to be
used in marine science.
B.2 Base Units
B.2.1 Length
The metre was originally proposed by the French Academy of Science in 1791
as one ten-millionth of the distance from the North Pole to the equator on the
meridian passing through Paris. However, the surveyors got their sums wrong
and for 162 years (to 1960) the metre was defined as an arbitrary distance
marked on a metal bar made of 90 percent of platinum and 10 percent of
iridium and preserved in the International Bureau of Weights and Measures at
Sevres, near Paris.
At present, one metre is defined as the distance travelled by light in a vacuum
in 1/299,792,458 second. This value results from the value of the speed of light
accepted in 1983 as 299,792,458 metres per second.
In oceanography, sailing and shipping, distance is frequently expressed in
nautical miles (Nm), where 1 nautical mile = 1852m. On older nautical maps,
water depth is not expressed in metres but in feet and fathoms where the
following relationships exist:
1 foot = 12 inches = 30.48 cm = 0.3048 m,
(B.1)
1 fathom = 6 feet = 1.83 m.
(B.2)
B.2.2 Time
In the past, the basis of time units were astronomical events. For example,
1 second was defined as 1/86400 of the time between two consecutive highest
positions of the Sun. According to the present definition, the second is equal
to 9,192,631,770 periods of the light emitted by atom of Caesium l33Cs. The
number of periods of radiation was chosen to make the length of the defined
second correspond as closely as possible to that astronomically determined.
At present, the most accurate timekeeping device is a commercially available
atomic clock which makes an error of one second in 1.6 million years.
Appendix B International System of Units
In SI system, the fundamental units are:
• length - metre (m),
• time - second (s),
• mass - kilogram (kg),
• temperature - kelvin (K).
Prefixes are attached to names or symbols of SI units to form powers-of-ten
multiples. Table B.1 lists the prefixes and associated SI symbols likely to be
used in marine science.
B.2 Base Units
B.2.1 Length
The metre was originally proposed by the French Academy of Science in 1791
as one ten-millionth of the distance from the North Pole to the equator on the
meridian passing through Paris. However, the surveyors got their sums wrong
and for 162 years (to 1960) the metre was defined as an arbitrary distance
marked on a metal bar made of 90 percent of platinum and 10 percent of
iridium and preserved in the International Bureau of Weights and Measures at
Sevres, near Paris.
At present, one metre is defined as the distance travelled by light in a vacuum
in 1/299,792,458 second. This value results from the value of the speed of light
accepted in 1983 as 299,792,458 metres per second.
In oceanography, sailing and shipping, distance is frequently expressed in
nautical miles (Nm), where 1 nautical mile = 1852m. On older nautical maps,
water depth is not expressed in metres but in feet and fathoms where the
following relationships exist:
1 foot = 12 inches = 30.48 cm = 0.3048 m,
(B.1)
1 fathom = 6 feet = 1.83 m.
(B.2)
B.2.2 Time
In the past, the basis of time units were astronomical events. For example,
1 second was defined as 1/86400 of the time between two consecutive highest
positions of the Sun. According to the present definition, the second is equal
to 9,192,631,770 periods of the light emitted by atom of Caesium l33Cs. The
number of periods of radiation was chosen to make the length of the defined
second correspond as closely as possible to that astronomically determined.
At present, the most accurate timekeeping device is a commercially available
atomic clock which makes an error of one second in 1.6 million years.