Qu'est-ce qu'un/une Fathom (ftm) ?
Formal Definition
The fathom (symbol: ftm) is a unit of length equal to 6 feet or exactly 1.8288 meters. It is used almost exclusively for measuring the depth of water, particularly in nautical contexts. One fathom equals 2 yards, 72 inches, or 1.8288 meters.
Body-Based Origin
The fathom originally represented the span of a man's outstretched arms — from fingertip to fingertip. This body measurement was naturally suited to hauling and measuring rope and anchor line aboard ships, as a sailor could measure line by stretching it between his outstretched arms, one fathom at a time.
Maritime Context
The fathom is fundamentally a maritime unit. It has been used for centuries to measure water depth, anchor chain length, and fishing line. The phrase "to fathom" (meaning to understand or get to the bottom of something) derives from this unit's association with plumbing the depths.
Etymology
Old English Roots
The word "fathom" comes from the Old English "fæthm" or "fæðm," meaning "outstretched arms" or "embrace." It is related to the Old Norse "faðmr" (embrace), the Old High German "fadam" (thread, fathom), and the Dutch "vadem." The Proto-Germanic root *fathmaz referred to the span of outstretched arms.
Metaphorical Use
The verb "to fathom" — meaning to understand something deeply or to get to the bottom of a mystery — dates from the early 17th century. It derives from the literal act of fathoming: lowering a weighted line to measure water depth. To fathom something is thus to measure its depth, literally or figuratively.
Related Words
The word is cognate with the Greek "πέταμα" (petama, something spread out) and the Latin "patere" (to lie open). These connections reinforce the original meaning of something spread wide — the outstretched arms.
Precise Definition
Modern Definition
The fathom is defined as exactly 6 feet, which equals exactly 1.8288 meters under the 1959 international yard and pound agreement. This is a precise, fixed definition with no ambiguity.
Historical Variations
Before standardization, the fathom varied across countries. The Danish fathom (favn) was about 1.883 m. The Swedish fathom (famn) was about 1.781 m. The French brasse was about 1.624 m. The Spanish braza was about 1.672 m. These variations reflected different arm-span traditions and different foot lengths.
Measurement Practice
Historically, depth was measured by "sounding" — lowering a weighted line (the sounding lead or lead line) to the bottom and reading the depth from marks on the line. The line was marked in fathoms using a system of leather strips, cloth, and knots at specific intervals. Modern echo sounders and multibeam sonar have replaced the lead line but the fathom persists as a unit.
Histoire
Ancient Origins
The fathom is one of the oldest units of measurement, with arm-span measurements documented in ancient Greece (the orguia, about 1.85 m), ancient Rome, and Viking Scandinavia. The unit was naturally suited to measuring rope and line — the fundamental technology of seafaring.
Medieval and Early Modern Use
In medieval England, the fathom was standardized at 6 feet and used extensively in maritime contexts. The Admiralty used fathoms for all depth measurements on nautical charts. Lead-line sounding, with depths called out in fathoms, was a critical skill for navigators entering unfamiliar waters.
Mark Twain Connection
The pen name of Samuel Clemens — "Mark Twain" — comes from the sounding call "mark twain," meaning two fathoms (12 feet). On Mississippi River steamboats, the leadsman would call out depths: "by the mark, twain" indicated a depth of two fathoms — the minimum safe depth for a riverboat. Other calls included "mark three" (three fathoms) and "no bottom" (depth exceeding the line).
Decline in Modern Use
With the global adoption of metric charts, the fathom has been gradually replaced by the meter for depth measurement. The International Hydrographic Organization (IHO) has promoted metric charts since the 1960s. However, US nautical charts continued to use fathoms and feet for bathymetric data well into the 21st century, and many older charts worldwide still show depths in fathoms.
Utilisation actuelle
Nautical Charts
Some US nautical charts published by NOAA still display depths in fathoms and feet, particularly for deep water. However, the US has been transitioning to metric charts, and new charts increasingly show depths in meters. International charts produced to IHO standards use meters.
Fishing Industry
Commercial fishing still uses fathoms in English-speaking countries. Net depths, fishing line lengths, and water depths at fishing grounds are commonly described in fathoms by American and British fishermen. A trawl net might be set at "50 fathoms" (300 feet, 91.4 m).
Literature and Culture
The fathom persists in literary and cultural references. Shakespeare's The Tempest includes "Full fathom five thy father lies" — one of the most famous uses of the unit. Jules Verne's Twenty Thousand Leagues Under the Sea uses fathoms and leagues for depth measurements. The unit adds a nautical atmosphere to maritime writing.
Anchor Chain
In the US Navy and merchant marine, anchor chain is still measured in fathoms. A standard "shot" of anchor chain is 15 fathoms (90 feet) long. Ships carry multiple shots of chain, and the amount of chain deployed is described in fathoms or shots.
Everyday Use
Limited Everyday Use
The fathom is rarely encountered in everyday life outside maritime contexts. Most people know the unit only from literature, the expression "I can't fathom it," or the pen name Mark Twain.
Boating and Diving
Recreational boaters in the US may encounter fathoms on older charts or depth sounders that offer fathom display options. Scuba divers generally use feet or meters rather than fathoms, but the term appears in diving literature and wreck descriptions ("the wreck lies at 20 fathoms").
Fishing
Recreational anglers, especially deep-sea fishermen, may use fathoms for describing depths. "Fishing at 40 fathoms" (240 feet) is more common among experienced fishermen than specifying the depth in feet.
In Science & Industry
Bathymetry
Historically, ocean depth measurements were recorded in fathoms, and vast amounts of bathymetric data exist in fathom-based units. The Challenger expedition (1872-1876), which conducted the first systematic survey of the ocean floor, recorded all depths in fathoms. Converting these historical measurements to meters is straightforward (multiply by 1.8288).
Oceanography
Modern oceanography uses meters exclusively, but historical oceanographic literature uses fathoms extensively. The deep ocean was described in fathoms well into the 20th century: the Mariana Trench's depth was historically given as about 6,000 fathoms (approximately 10,973 meters).
Geology
Some geological and mining contexts, particularly in the UK and US, have historically used fathoms for depth measurements. Cornish tin mines, for example, measured shaft depths in fathoms. This usage is now obsolete, replaced by meters.
Interesting Facts
Samuel Clemens chose the pen name "Mark Twain" from the Mississippi riverboat sounding call meaning two fathoms (12 feet) — the minimum safe depth for a steamboat. This was called out by the leadsman as "by the mark, twain."
Shakespeare wrote in The Tempest: "Full fathom five thy father lies; Of his bones are coral made." Full fathom five (30 feet or 9.14 meters) is a moderate depth — enough for a body to remain on the seafloor undisturbed.
The Mariana Trench, the deepest point in the ocean, is approximately 6,010 fathoms (10,994 meters or 36,070 feet) deep at the Challenger Deep.
A standard shot of anchor chain in the US Navy is 15 fathoms (90 feet). An aircraft carrier might carry 12 shots per anchor — 180 fathoms (1,080 feet) of chain.
The lead line used for sounding had distinctive markings: 2 fathoms was marked with leather strips, 3 fathoms with leather, 5 fathoms with white linen, 7 fathoms with red bunting, 10 fathoms with leather, and so on.
Jules Verne's novel "Twenty Thousand Leagues Under the Sea" (1870) uses the league as a unit of distance, not depth. The title refers to the distance traveled horizontally, not the depth, though fathoms are used for depth measurements in the story.
The expression "to fathom" (to understand deeply) first appeared in English around 1600, metaphorically extending the literal meaning of measuring depth to measuring understanding.
Regional Variations
United States
The US has been the last major maritime nation to transition from fathoms to meters on nautical charts. NOAA charts have traditionally shown depths in fathoms and feet, but newer electronic charts increasingly use meters. The US fishing industry continues to use fathoms.
United Kingdom
The UK Hydrographic Office switched to metric charts decades ago, and fathoms are rarely used in professional British maritime contexts. However, the term survives in British English expressions and literature.
International
The International Hydrographic Organization promotes metric charts worldwide, and most countries have completed the transition. Fathoms appear on older charts still in use and in historical maritime literature.