Dan Marino, one of the greatest quarterbacks in the history of the National Football League, wore No. 13 throughout his career with the powerhouse Miami Dolphins. Marino threw for an astounding 61,358 yards and 420 touchdowns, but he never won a Super Bowl.

When the team honored Marino by retiring his famed jersey during a game Sept. 17, a south Florida downpour lashed the ceremony. Certainly, Marino was not triskaidekaphobic.

Perhaps he should have been.

Triskaidekaphobia is a fear of the number 13, and it is a source of fascination to Thomas J. Fernsler, an associate policy scientist in the Mathematics and Science Education Resource Center who, in the world of mathematics, is known as "Dr. 13."

Fernsler says he first was struck by the unusual power of the number 13 in 1987, which, of course, was 13 years ago.

"It was the first time I had noticed three Friday the 13ths in one year," Fernsler says. "I thought, that seems like a lot. Then, I started reading everything I could about the subject."

He found that, from earliest times through this, the modern information age, the number 13 has had a special mystique. "It's the one superstition that doesn't seem to let go," Fernsler says. "This is the year 2000. Who's superstitious anymore? But, you'd be surprised how many people are."

Napoleon, J. Paul Getty, Herbert Hoover and Franklin Delano Roosevelt were all practicing triskaidekaphobes, Fernsler says.

"FDR might have been our most superstitious president," he says. "He was scared to death of the number 13. When luncheon or dinner parties numbered 13, he would ask his secretary to join the guests to make an even 14."

That is a common fear, Fernsler says, noting that, in Paris, superstitious diners can hire a quatorzieme, or professional 14th guest.

Mark Twain once was the 13th guest at a dinner party, Fernsler says, and a friend told him not to go because it was bad luck. "It was bad luck," Twain later told the friend. "They only had food for 12."

Roosevelt's fears extended far beyond the dinner table, affecting even his travel arrangements. "If he was going to travel on the 13th, often he would make the conductor leave at 11:50 p.m. on the 12th or wait until the early hours of the 14th," Fernsler says.

A more recent example is the nearly tragic flight of Apollo 13, which was recreated in a movie directed by Ron Howard and starring Tom Hanks.

Fernsler says the screenwriters noted obvious references to the number, such as the mission number, the fact that liftoff was at 1313 hours CST and that the explosion that crippled the spacecraft occurred on April 13. Two points they missed, he says, were the fact that the sum of the digits in the numeric launch date of 4-11-70 is 13 and that the rocket was fired from launch pad 39, the third multiple of 13.

Fernsler says 13 suffers from its position after 12, which numerologists consider a complete number. There are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel and 12 apostles of Jesus.

In exceeding 12 by one, Fernsler says, 13 "is just beyond completeness, and is, therefore, restless and squirmy."

People get particularly squirmy when 13 falls on a Friday, Fernsler says, because that day has a reputation as one of bad luck.

Fernsler notes that every year contains at least one Friday the 13th. The most that can occur in any one calendar year is three; the last time that happened was 1998, and the next will be 2009. If you're feeling nervous, there will be two in 2001--April and July.

Fernsler works with teachers through UD's Mathematics and Science Education Resource Center, which helps educators implement new curriculum content and performance standards in their classrooms

By appearing as Dr. 13, Fernsler has an opportunity to spread the word that, indeed, math can be fun.

"I enjoy doing this, as long as people have an interest in it," he says, "and they seem to."