Fearful Friday the 13th

Lucky 2009 has three Fridays the 13th in one year. Three in one year happens once every 11 years. The first was in February, second one is today, and the last one is in November.

The connection between bad luck and Friday the 13th is not very clear. Perhaps it has links to the 13th guest at the Last Supper betraying Jesus; but, regardless of where it started, it has become a superstition with a life of its own.

Here are five Friday-the-13th facts:

1. Fear of Friday the 13th is called paraskavedekatriaphobia as well as friggatriskaidekaphobia. Triskaidekaphobia is fear of the number 13.

2. Many hospitals have no room 13, some tall buildings have no 13th floor and some airline terminals have no Gate 13.

3. President Franklin D. Roosevelt would not travel on the 13th day of any month (Friday or otherwise) and would never host 13 guests at a meal. Napoleon and President Herbert Hoover were also triskaidekaphobic, with an abnormal fear of the number 13.

4. Mark Twain once was the 13th guest at a dinner party. A friend warned him not to go. "It was bad luck," Twain later told the friend. "They only had food for 12." Superstitious diners in Paris can hire a quatorzieme, or professional 14th guest.

5. The number 13 suffers from its position after 12, according to numerologists who consider the latter to be a complete number. The number 12 is seen everywhere: 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel, 12 apostles of Jesus, 12 days of Christmas and 12 of anything in a dozen. The number 13 appearing one number after this seems incomplete.

Meanwhile the belief that numbers are connected to life and physical things — called numerology — has a long history.

"You can trace it all the way from the followers of Pythagoras, whose maxim to describe the universe was 'all is number,'" says Mario Livio, an astrophysicist and author of "The Equation That Couldn't Be Solved" (Simon & Schuster, 2005). Thinkers who studied under the famous Greek mathematician combined numbers in different ways to explain everything around them, Livio said.