On the morning of 9 September 1947, operators working on the Harvard Mark II — a five-ton electromechanical calculator built for the U.S. Navy’s Bureau of Ordnance — traced a fault to Relay #70 in Panel F. Inside the relay, wedged between the contacts, was a moth. Someone extracted it with tweezers, taped it to a page of the operations logbook, and wrote in careful pencil: “First actual case of bug being found.” The page, moth still attached, is preserved today in the Smithsonian’s National Museum of American History.
It is one of the most beloved artefacts in the history of computing, and the story around it is almost entirely misunderstood. The moth did not give the world the word “bug.” The joke in that logbook entry only lands if you already know that engineers had been calling faults “bugs” for at least seventy years before a single moth wandered into a Navy relay.
What the logbook actually says
The Mark II was the successor to Howard Aiken’s Harvard Mark I, a room-sized programmable calculator that had helped run numerical simulations for the Manhattan Project. The Mark II used high-speed electromechanical relays rather than the Mark I’s rotating shafts, and it was installed at the Naval Proving Ground at Dahlgren, Virginia, though the September 1947 debugging session took place while the machine was still being shaken down at Harvard.
The logbook page for 9 September records a series of test runs on the machine’s multiplier and adder circuits. Between entries timed at 15:45 and 16:00, someone recorded that Relay #70 in Panel F had been found faulty. Below that line, taped to the page, is the moth. The handwritten annotation reads: “First actual case of bug being found.” The wording is precise. Not “first bug.” Not “we invented a new term today.” First actual case — as in, the first time the metaphor turned out to be literal.
The identity of the person who taped the moth in place is not certain. The find is often attributed to Grace Hopper, then a Navy Reserve lieutenant working on the Mark II programming team, and Hopper enthusiastically retold the story for the rest of her life. But Hopper herself was consistent that she was not the one who discovered it, and the handwriting in the log has been attributed by the Computer History Museum and other historians to William “Bill” Burke of the Naval Surface Warfare Center. Hopper was the story’s narrator, not its protagonist.
Where “bug” actually came from
By 1947, “bug” as engineering slang for a defect was already old. The Oxford English Dictionary traces the sense — a fault or flaw in a piece of machinery — back to the 1870s. The most-cited early source is Thomas Edison, who used the word repeatedly in his notebooks and correspondence to describe the frustrating small faults that appeared when a new invention was being coaxed into working order.
In an 1878 letter to Theodore Puskás, reprinted in the Thomas A. Edison Papers at Rutgers, Edison wrote that difficulties with a new device were nothing unusual: “This thing gives out and [it is] then that ‘Bugs’ — as such little faults and difficulties are called — show themselves.” The quotation marks around “Bugs” are Edison’s own, and they tell you the word was already in currency among engineers — enough that Edison felt he was using shop slang, not coining a term.
The metaphor spread quickly. By the early twentieth century, aircraft engineers, telephone technicians, and radio operators all used “bug” and “debug” to describe hunting down intermittent faults. A 1944 issue of the Journal of the Royal Aeronautical Society referred to the process of “debugging” an aeroplane’s systems in exactly the modern sense. When the Harvard team wrote “first actual case of bug being found,” they were leaning on a joke every engineer in the room would have understood instantly.
Why the story stuck anyway
Origin myths are sticky because they are neat, and the moth story is very neat. A room-sized computer. A dead insect. A tidy piece of physical evidence you can point at in a museum case. It compresses a long, diffuse history of engineering slang into a single dated event with a photograph, and it flatters the intuition that words for small annoying things ought to come from small annoying things.
The truth is messier and more interesting. “Bug” survived and thrived in computing not because a moth was pulled from a relay in 1947, but because the metaphor was already perfect for the work. Early computers — vacuum tubes, relays, hand-soldered wiring — failed constantly and unpredictably. A loose contact, a dying tube, a stray cosmic ray flipping a bit: each of these behaved like a small living thing that had crawled into the machine and would have to be tracked down and removed. Programmers inherited the language because they inherited the experience.
Grace Hopper’s role in the myth is worth noting too. Hopper was one of the twentieth century’s great communicators of computing, a compiler pioneer who spent decades explaining what computers were and how they worked to audiences that had never seen one. She used the moth story in lectures because it was funny and it stuck, and because it let her explain a technical culture — the culture of debugging — to people who had never written a line of code. The story became famous partly on the strength of her telling.
The moth today
The logbook page, moth included, was retained by the Naval Surface Warfare Center Dahlgren Division for decades before being transferred to the Smithsonian in 1988. It is catalogued under accession number 1994.0191.01 and occasionally goes on display at the National Museum of American History. The insect itself, remarkably, is still there — flattened, brown, slightly crumbled at the wingtips, taped down with cellophane that has yellowed over three-quarters of a century.
What the artefact really documents is not the birth of a word but a moment of engineers being funny at 4pm on a Tuesday. They were debugging a computer. They found, for once, an actual bug. They thought it was worth writing down, and they were right, though not for the reasons the popular version of the story suggests. The joke has outlived everyone in the room.
It has also outlived the machine. The Mark II was decommissioned in the early 1960s and largely dismantled; only fragments survive in collections. The moth, pressed between the pages of an ordinary bound logbook, has done considerably better. It is a small reminder that in the history of technology, the artefacts that endure are rarely the ones the engineers thought were important at the time. They are the ones somebody bothered to tape down.
