Many people around the world thought Saturday, July 7th, 2007, might be the luckiest day of the century. That's because in the Gregorian Calendar, it was the seventh day of the seventh month of the seventh year. This might explain why about three times as many weddings were reported that day in the U.S. as for a typical Saturday in July. On the other hand, the 13th day of July falls on a Friday – a coincidence many believe brings bad luck. Why are these numbers so meaningful to some people?
Ian Stewart is a British mathematician and author at the University of Warwick who also publishes an Internet web log on popular mathematics. He recently wrote an article on number symbolism for the Encyclopedia Britannica online.
There is a cultural fascination with numbers, Stewart says, and people tend to give them all sorts of significance. He notes that the number 13 is unlucky for many Western cultures, but 4 is unlucky for the Chinese, because the Chinese word for four and the Chinese word for death sound very similar. It varies from culture to culture, Stewart says, all of which suggests that the numbers themselves don't actually have these particular features.
Surprisingly, the number seven is consistently considered lucky, even for very different cultures. Everyone knows that in western casinos, a person wins at the slot machines if they get triple sevens, and seven is the most probable outcome of rolling two dice. But even the ancient Greeks thought seven was a special number.
Stewart explains that the Pythagoreans in ancient Greece thought the number 3 represented the spiritual world. They associated the number 4 with the four essential elements in the physical world: fire, earth, air and water. Therefore, 7 – which was 4 plus 3 – was particularly important, he says, because it embodied both the spiritual world and the material world.
According to Stewart, the Chinese also have a special reverence for the number seven. They believe that many stages of their lives are related to that number, he explains. People get their baby teeth when they're seven months old, their adult teeth when they're seven years old, go through puberty when they're 14, which is 2 times 7, women go through menopause when they're 49, which is 7 times 7. Stewart says people find these patterns significant because they suggest, in one sense, a grand plan. Clearly, he notes, these ages aren't exact, women don't always go through menopause at 49, but they're very close approximations. And the number seven keeps coming up.
Stewart notes, too, that the lunar cycle is 28 days long, which is 7 times 4. He says that this could be a reason why, in so many different cultures, weeks are seven days long. For the ancient Babylonians, he says, seven was an especially important number in astronomy. They believed there were seven heavenly bodies that moved around the planet, against a background of fixed stars: Mercury, Mars, Venus, Jupiter, Saturn, the Sun, and the Moon. One of the ways we like to order our world, Stewart observes, is to look at things we see in nature, associate numbers with them, and then say 'Ah! See? Those numbers are important.'
He thinks there might also be a more mathematical reason behind our interest in numbers like 7 and 13: both are prime numbers. He says these numbers are surprising because they can't be built up from smaller numbers by multiplying them together. Every time you come to a prime number, Stewart explains, you think: this is sort of different from everything that's come before, it's not related to the numbers I understand, so perhaps it has all sorts of strange properties.
Maybe there is an explanation for some of these things, says the author, but it's not that particular numbers are lucky or unlucky. "It's the way we react to them," Stewart says. "We tend to invent patterns even when they don't really exist because our brains like patterns."
As a mathematician, Ian Stewart doesn't believe numbers have any supernatural powers. But he understands their emotional appeal, which is probably why days like July 7th, 2007 and Friday, July 13th are special for many people, and why a faith in the symbolic power of numbers has been such an enduring part of human culture.