For eight decades, to borrow a Roman adage, Christine F. Sleeper has been seizing the day. Even the license plate on her old Pontiac speaks to her enthusiasm. It reads "GAUDE," the Latin word for "enjoy," and before you can say, "Mirabile Dictu!" she conjugates the verb: "Gaudeo, Gaudes, Gaudet, Gaudemus, Gaudetis, Gaudent."

Sleeper turns 80 (LXXX to her) tomorrow, and she will mark the event at Herndon High School, where she remains a full-time Latin teacher.

"I remember a couple of years ago at a faculty meeting, the principal said it was my birthday and someone down at the back whispered, Why the hell is she still teaching?' " Sleeper said.

"I love it. I love the opportunity to teach and to show students what a wonderful intellectual experience Latin can be. We can learn so much from ancient thought and how we have progressed since then."

Blessed with Katharine Hepburn good looks, and the presence to match, Sleeper was the magistra of her classroom yesterday as she roamed among the desks, thrilling to correct answers and smiling with a maternal forgiveness when her students stumbled.

"She's great," said Dax Gray, a 17-year-old senior. "She looks likes she's 21 and she teaches like she's 21. Every day she's enthusiastic and excited."

As she hands out homework, she tells her charges that they are sure to enjoy the exercise: Find the Latin pronoun that corresponds to the English pronoun in italics in a series of sentences otherwise written in Latin. Then translate the sentences into English.

"It's a nice challenge," she says, in a phrase that has always found an echo in her own life, both in and out of the classroom.

Sleeper, who grew up in New Hampshire, began teaching at a private school as soon as she got her master's degree from Radcliffe College. But within a couple of years, she decided she wanted to become a pilot instead.

"I'm always looking for new things to feel alive," she said, before pausing to recite from Tennyson's "Ulysses":

I am a part of all that I have met;

Yet all experience is an arch wherethro'

Gleams that untravell'd world whose margin fades

For ever and for ever when I move.

During World War II, she worked as an air traffic controller and later served with the American Red Cross in Europe. She returned to the United States in 1946 and married Raymond Sleeper, an Air Force officer. Over the next 20 years, bouncing from military post to post, she raised six children and had no time for teaching or piloting.

In 1965 she returned to teaching. And when her husband retired from the military in 1969, the couple moved to Herndon and Sleeper took a position at Herndon High School.

It was not the most auspicious time to be a Latin teacher. Interest in Latin fell precipitously from the early 1960s through the mid-1970s, when the number of U.S. high school students taking the language dropped from 700,000 to 150,000, according to the American Classical League, a teachers association. Those numbers have slowly crawled back up, and now 490,000 students, kindergarten through 12th grade, study Latin.

"Latin is enjoying a renaissance," said Richard LaFleur, professor of Latin at the University of Georgia and past president of the American Classical League. "Students and parents are recognizing that it builds English vocabularly and is a bridge to the Romance languages. That's the big-picture reason. And then you have powerhouses like Christine who inspire students."

And her students agree.

"She loves Latin," said Sara Baker, 15, a 10th-grader who is taking her second year of Latin. "You take it because it helps you on the verbal in the SAT, and you expect it to be dry, but it never is with her."

Sleeper suffered a heart attack last year and missed a semester. But just last week, she won a gold in the 50-meter breast stroke and a bronze in the half-mile race in the Northern Virginia Senior Olympics. "I had to play hooky to compete," she said.

Tomorrow the school will wish her "Tibi Felicem Natalem Diem" ("To you happy birth day") with a faculty party and a declaration from the town of Herndon that Sept. 20, 1996, is Christine F. Sleeper Day.

Although the Fairfax County school system has no mandatory retirement age, Sleeper acknowledges she has thought about quitting from time to time.

"But there's always a new group of students, and you spot one who is engaged and alert and fascinated," she said. "And I think, I want to teach that student. I have to teach that student.' "

CAPTION: LXXX Going on XXI

Christine F. Sleeper, who turns 80 tomorrow, has taught Latin at Herndon High since 1969. Her view of teaching: "I love it."