Dear Straight Dope:
In aviation, the phrase "dead reckoning" refers to a primitive form of aircraft navigation, solely by reference to compass headings and elapsed time. A majority of pilots insist that the correct spelling of the phrase is "ded reckoning," a contraction of "deduced reckoning." To me that sounds like a stupid way for old-timey aviators to talk, and I don't buy it. However, no one seems to have any proof one way or the other. That is, until now. Hook us up.
What we have here is a battle. On the one side we have a combined force of tough sea dogs and intrepid flyboys, and on the other, a bunch of nerdy dictionary geeks. Needless to say, I’m with the geeks. The other side may have superior firepower, but the etymologists have history on their side.
First, a definition is in order. Dead reckoning is the process of estimating the position of an airplane or ship based solely on speed and direction of travel and time elapsed since the last known position (or fix). So all you need to figure out approximately where you are is an airspeed indicator or log or other measure of speed, a clock or watch, and a compass. Dead reckoning stands in contrast to pilotage (navigation by visible landmarks) and celestial navigation (navigation by reference to stars or other heavenly bodies). Since the development of radio technology, various forms of electronic navigation have also been developed, the best known of which is the satellite-based Global Positioning System. Navigating by external reference points is more accurate, but dead reckoning is the fallback when all else fails.
The term dates from the seventeenth century, so we have to look to the sea for the origin of the term, not the air. The story you refer to has it that dead reckoning was first called “deduced reckoning,” which someone then abbreviated (in a ship’s log) as “ded. reckoning.” Later someone reading it thought “ded” didn’t make much sense, so he wrongly (according to this theory) thought it must be a misspelling for “dead.” The other theory is that it was “dead reckoning” from the beginning, but since this sort of navigation doesn’t seem to have much to do with death, someone assumed the derivation from “deduced,” which must have made more sense to him. In either case, folk etymology is at work, but it isn’t immediately obvious which is the real etymology and which is the folk etymology.
If my searches of the Internet and Usenet are any indication, the derivation from “deduced” is familiar to most U.S. pilots, and accepted by many of them. It is also known among U.S. sailors, though perhaps not quite as widely (or perhaps they’re just less inclined to repeat it). But in other English-speaking countries (including Canada), this derivation seems to be largely unknown.
Of all the dictionaries I checked that gave an etymology, the American Heritage Dictionary is most amenable to the “deduced” theory, saying “Possibly alteration of ded. abbr. of deduced.” “Possibly” is the kindest treatment this theory is given in any dictionary I could find. The Encarta World English Dictionary says “probably from dead ‘absolute’ or ‘exact,’ although ‘dead’ may be by folk etymology from ded., a shortening of deduce or deduction.” The Dictionary of Misinformation says of the “deduced” theory, “There is no evidence for such a belief.” The Oxford English Dictionary says that the term is from the adjective “dead” and doesn’t deign to even discuss the supposed derivation from “deduced”. The Oxford Dictionary of English Etymology provides the final nail in the coffin: “a proposed etym. ded., for deduced, has no justification.”
These sources agree on the derivation from the adjective “dead” but differ on what “dead” is supposed to mean in this context. One theory, supported by the OED, is that it’s dead in the sense “complete(ly)” or “absolute(ly),” also found in “dead wrong,” “dead ahead,” “dead last,” etc. The idea seems to be that the dead-reckoning position is one based completely on reckoning (calculation) and not at all on observation of landmarks. Others hold that “dead” means “unmoving,” as in “dead in the water.” The idea here is that dead reckoning is calculated with respect to an object (like the log) that is dead in the water (not moving with respect to the surface of the water). Yet another theory is that it comes from “dead seas” (supposed to mean “unknown seas”), where dead reckoning would be an important tool.
Lining up against the dictionaries are the navigation manuals, many of which present the derivation from “deduced” as true or probably true. The earliest known reference to this theory was pointed out to me by a helpful reader after this report was first published. It is in Avigation (1931) by Bradley Jones. Other early purveyors of the theory include: Marine and Air Navigation (1944), The Bluejackets’ Manual (thirteenth edition, 1946), Dutton’s Navigation and Nautical Astronomy (tenth edition, 1951), The Encyclopedia of Nautical Knowledge (1953), and a great many later books. Bowditch’s American Practical Navigator (1958 edition) splits the difference and counts both derivations as true (more on this later). I include the dates of publication to point out the fact that although there is no shortage of sources repeating the theory that date from during and after the Second World War, I could find no reference to the theory in any earlier sources, despite checking well over a hundred books on navigation in six different libraries. Where possible, I checked earlier editions of the books mentioned above. For example, I checked six earlier editions of Bowditch, but found no etymological information in any of them.
Books about flying and sailing are by no means unanimous in accepting the “deduced” theory. The Oxford Companion to Ships and the Sea says of it, “this origin of the term is improbable; it has too much of a modern ring about it.” The Norton Encyclopedic Dictionary of Navigation supports the derivation from the adjective “dead” as in “dead in the water.” In an FAA pamphlet titled “Dead Reckoning Navigation,” aviation writer Barry J. Schiff agrees, saying “According to popular definition, dead reckoning is short for ‘deduced reckoning’ or, as the old-timers used to say, ‘you’re dead if you don’t reckon right.’ In truth, however, the term originated with maritime navigation and refers to ‘reckoning or reasoning (one’s position) relative to something stationary or dead in the water.'”
The lateness of any sources that mention “deduced reckoning” or “ded. reckoning” stands in contrast to the early use of “dead reckoning.” The first citation in the Oxford English Dictionary for “dead reckoning” (spelled thus) is from 1613. In fact, in all of the OED‘s citations, the modifier is always spelled “dead.” In the earliest dictionary entry I could find for the term (1708, Dictionarium Anglo-Britannicum), it is spelled “Dead-reckoning” (with the hyphen, but without any mention of “deduced”). It seems that the “dead” spelling is older than “ded.” by more than three centuries.
But the supporters of “ded.” are not done yet. Bowditch’s American Practical Navigator (1958 edition and some later editions) splits the difference by making both etymologies right. According to this theory, “dead reckoning” in nautical use is properly restricted to mean reckoning relative to something that is dead in the water, taking no account of current and leeway. In contrast to the dead reckoning (DR) position, a reckoning that does take leeway and current into account is now usually called the “estimated position” (EP). The distinction between DR and EP goes back at least to the 1920s, and probably earlier. According to the theory laid out in Bowditch, EP reckoning was originally called “deduced reckoning,” (abbreviated “ded. reckoning), which is supposed to have originated independently of “dead reckoning.” The two terms were then confused for obvious reasons. It’s a neat idea if true, but I have my doubts — I’ve seen no evidence of “deduced” or “ded.” being used this way before 1931. (For what it’s worth, the theory has been removed from more recent editions of Bowditch — see the online edition of 1995 in PDF format.) Even if “ded reckoning” were found in some seventeenth-century ship’s log, it wouldn’t necessarily mean the “deduced” derivation is right. “Ded” was a common spelling for “dead” in those days before English orthography was standardized. That may be the ultimate origin of the confusion, but I have no evidence to support this idea.
As noted above, the earliest references I could find to the “deduced” theory date to dates to the decade before the Second World War. Possibly the theory was floating around for some years before that, but if so it could not have been very widely known. No mention is made of it in the American Speech article “Our Heritage of Old Sea Terms” by George Wasson (1929), even though he defines “dead reckoning” and discusses other disputed etymologies. This leads me to believe the “deduced” etymology was unknown to Wasson in 1929. As for “old timey aviators,” Charles A. Lindbergh spelled it “dead reckoning” in both the books he wrote about his historic flight (published in 1927 and 1953). If “dead” is good enough for Lindbergh, it’s good enough for me.
The WWII-era popularization of the etymology accords with my personal experience. I was first exposed to the “deduced” derivation by my late father, who had been a flight instructor early in the war. At the time I was young and impressionable and didn’t question him. Unfortunately, I never thought to ask him when and where he first learned this. Since the war, this etymology’s popularity has waxed and waned, but it is now almost dogma for many aviators. It used to be mostly people with a real interest in navigation, like student pilots, who were exposed to the “deduced” theory. In the computer age, it has infected millions of people who couldn’t navigate their way out of a well marked garage. I became skeptical a couple years ago when I bought flight simulator software for my computer, which presented the “deduced” theory as fact. A quick check of the dictionary was all it took to convince me the software (and my father) were probably wrong all along.
Some feel compelled to “correct” people who write “dead” and not “ded.” Obviously I don’t believe they should, but it might come as a surprise to some to learn that neither does the FAA or the Coast Guard. A search of their websites finds zero references to “ded” or “ded.” or “deduced” reckoning, but quite a few references to “dead reckoning.” In practice, the term is usually abbreviated DR, which is probably safest if you don’t want to get into an argument.
I feel compelled to note that in addition to dead reckoning there’s also dud reckoning, the system I used that enabled me to get lost on the way to one of the libraries I visited. So maybe you don’t want to make me the navigator on your next polar expedition. When it comes to etymology, though, I say stick with me.
Send questions to Cecil via firstname.lastname@example.org.
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