History of October 10: 42 Day


  • Question: What is the answer to the ultimate question of life, the universe, and everything?
  • Answer: 42


Question: What does that mean?

I met Douglas Adams about 30 years ago when he spoke to Sun Microsystems, where I worked. I have never heard anyone talk quite as fast as he did.


Douglas Adams was famous for creating The Hitchhiker’s Guide to the Galaxy, a comedy science fiction series. First, it was available as a radio broadcast on BBC, but later adapted to several novels, a TV series, a feature film, and a video game.

His trilogy of 5 books, plus a 6th posthumously, features an episode where our protagonist posits a question to the computer “Deep Thought,” which spends 7.5 million years calculating. Such an incredibly simple and succinct answer to an amazingly profound question leaves one wondering what the answer “42” means. He gets the response:

“What do you get if you multiply six by nine?”
“Six by nine. Forty two.”
“That’s it. That’s all there is.”
“I always thought something was fundamentally wrong with the universe.”

The fact that six times nine is 54 does not deter our hero in his quest. Nor does the fact that in Base 13, this equation actually works. In the 1978 episode of the radio series, the Narrator announces:

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory mentioned, which states that this has already happened.


The author was repeatedly asked over the years for an explanation for why he picked the number 42. He replied that it is

“a completely ordinary number, a number not just divisible by two but also six and seven. In fact it’s the sort of number that you could, without any fear, introduce to your parents.”


Question: Why today, October 10?

In the binary system of numbering — that is, Base 2, which is all 1’s and 0’s — the number 42 comes out as 101010. That would be October 10, 2010 (or any century, actually), leading some fans to celebrate “42 Day” on any October 10th. This celebration is not, however, as widely practiced as Towel Day, a geek celebration held on May 25, the anniversary of the passing of Douglas Adams in 2001, as I explain here.


Question: Is that the end of 42 Day?

H2G2 1st EditionOf course not. Stephen Fry, a friend of Adams, claimed that the full answer was explained to him, that it was

“fascinating, extraordinary and, when you think hard about it, completely obvious.”

But he had vowed not to disclose it and instead would take it to his grave. Many scientists and mathematicians found Adams’ answer unconvincing. Video games, TV shows, movies, and even rock music songs all pay homage to the number 42.


Scientific American magazine published an article for math fans that goes into more depth on this mystery:

  • The Marathon distance is 42.195 kilometers
  • The Gutenberg Bible has 42 lines of text per column, aka the “Forty-Two-Line Bible”
  • It is the sum of the first three odd powers of two—that is, 21 + 23 + 25 = 42
  • It is the sum of the first two nonzero integer powers of six—that is, 61 + 62 = 42
  • It is a Catalan number that describes how many different ways an n-sided convex polygon could be cut into triangles by connecting vertices with line segments: 1, 1, 2, 5, 14, 42, 132…
  • It is a “practical” number, which means that any integer between 1 and 42 is the sum of a subset of its distinct divisors: 1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 28, 30, 32, 36, 40, 42, 48, 54, 56, 60, 64, 66 and 72

Of course, most of this is intuitively obvious to the most casual observer.


Conclusion to 42

In parting, one of my favorite lines from the book:

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”― Douglas Adams, The Hitchhiker’s Guide to the Galaxy


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Bill Petro, your friendly neighborhood historian

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About billpetro

Bill Petro writes articles on history, technology, pop culture, and travel. He has been a technology sales enablement executive with extensive experience in Cloud Computing, Automation, Data Center, Information Storage, Big Data/Analytics, Mobile, and Social technologies.

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