Gwen Thoma, director of educational services at Southeast Missouri Hospital, ought to have severe acne, erratic mood swings and a devotion to musical groups like the Backstreet Boys. After all, she turns 13 years old today.

And young Alexandra Wallace of Jackson insists she's not a baby, even though she is barely 1-year-old.

Thoma and Wallace are members of an elite group of people whose birthdays only come around once every four years. They all were born on Feb. 29, Leap Day.

"I feel real special when it comes around, but I feel saddened when it leaves because I know it won't be back for four years," said Thoma.

It's not easy to recognize a leap year when you see one. Most people think it comes every four years, but that's not true. Leap years are based on a mathematical equation created more than 2,000 years ago that mathematicians are still trying to perfect.

To understand what a leap year is, you must first understand the science behind development of modern calendars.

The modern calendar is based on the length of a tropical year. A tropical year is defined as the interval between two passages of the sun through the vernal equinox, that is, the instant the sun is above the Earth's equator while going from the south to the north. That length of time is 365.24219 days.

In 46 B.C., Julius Caesar created a 365-day calendar that included an extra day every four years. The addition of leap years created an average 365.25 day calendars.

It may not seem like a big difference, but over the years, the difference between the tropical year length and the Julian calendar length added up, and the seasons were off by some 10 days. Over time, that calendar would have seen the Fourth of July being celebrated in the middle of winter!

By 1582, Pope Gregory XIII set about creating a new calendar. The Gregorian calendar eliminated proposed century leap years that were not also divisible by 400. For example, 1700 and 1800 were not leap years in the Gregorian calendar, but 2000 is a leap year.

The changes proposed by Pope Gregory XIII meant the average year length was 365.2425 days, still not equal to the length of a tropical year, but close enough that there would not be a noticeable effect on the timing of the seasons for many thousands of years.

Because of Pope Gregory XIII's changes, leap year babies are looking forward to the first century leap year in 400 years.

"It's kind of exciting," said Trika Wallace, Alexander's mother. "We haven't had any problems generally, but when somebody asks her birthday it kind of throws them when you say on the 29th of February."

Wallace said being born on leap day could have its benefits for her daughter as she ages. "I could see when she gets to that 30-year old mark that could be pretty handy," she said.

For now, Alexandra Wallace usually celebrates her birthday on the weekend closest to Feb. 29 in non-leap years, while Thoma said she usually marks her birthday Feb. 28. That wasn't the case when Thoma was younger, though.

"A couple of times when I was younger I celebrated at midnight because I'd say my birthday was in there somewhere," she said. "I don't do that anymore. I figure we still grow old in the same number of years, so we just have to deal with it."

Why we have leap years

One year is approximately 365.24219878 days. The Egyptians called it 365 and left it at that. But their calendar got out of step with the seasons so that after around 750 years, they would be celebrating the Fourth of July in the middle of winter.

The Romans wised up and added the leap day every four years to get the 365.25 day Julian year. This time the year is longer than it ought to be. The small difference between this and the true length of the year caused the seasons to creep through the calendar once again, only slower and in the other direction.

By 1582 the calendar was about 10 days out of whack, so Pope Gregory XIII included the correction that's still in use today. If the year is divisible by 100, it's not a leap year unless it is also divisible by 400.

More recently, proposals for fixes include that if the year is also divisible by 4,000, it's not a leap year.

EGYPTIAN CALENDAR

Years to get 6 months out of whack: 754

JULIAN

CALENDAR

Years to get 6 months out of whack: 23,377

GREGORIAN CALENDAR

Years to get 6 months out of whack: 606,272

MODERN

CALENDAR

Years to get 6 months out of whack: 3,565,426

How to tell

if it's leap year

Even decades have three leap years:

1940 1944 1948

1960 1964 1968

1980 1984 1988

Odd decades have two leap years:

1952 1956

1972 1976

1992 1996

Age chart

If you were born on leap day, here's a comparison of your ages:

BIRTH LEAP ACTUAL

YEAR AGE AGE

1896 25 104

1904 24 96

1908 23 92

1912 22 88

1916 21 84

1920 20 80

1924 19 76

1928 18 72

1932 17 68

1936 16 64

1940 15 60

1944 14 56

1948 13 52

1952 12 48

1956 11 44

1960 10 40

1964 9 36

1968 8 32

1972 7 28

1976 6 24

1980 5 20

1984 4 16

1988 3 12

1992 2 8

1996 1 4