How important is a second?

To New York City officials, it's a "cosmic moment." To D.C. officials, it's a fleeting second. And to officials at the U.S. Naval Observatory, where the nation's master clock is maintained, it is a major delay in the beginning of the New Year.

Because the earth's rotation and man-made clocks are a second apart in time, an extra second, called a "leap second," will be added to the official time at the end of this year, according to Gernot Winkler, director of the Time Service Department at the Observatory. That adjustment will be made at 6:59:60 p.m., Eastern Standard Time on New Year's Eve, about five hours before the midnight countdowns in the District and New York.

So what do you do about the countdown?

Celebrators in New York will count down: 10-9-8-7-6-5-4-3-2-1-Leap-0. The leap second will feature -- briefly -- a light show around Times Square.

"We are putting the second in at the very end of 1987," said Tama Starr, whose company, Artkraft Strauss Sign Corp., is handling the New Year's Eve festivities. "This is a cosmic moment. It doesn't belong to '87. It doesn't belong to '88. It's an extra second in your life. Your whole life can change in a second. Think of some important moment when you said yes or no."

Naval Observatory astronomer Dennis McCarthy views it only slightly less cosmically. "A second is a relatively long time," he said. "If you're flying a plane by instruments and you're off by one second, you're going to miss the runway by nearly one-fifth of a mile."

In Washington, the leap second already will have been absorbed into the official party clock by the time the countdown begins at the Old Post Office Building, according to Vickey Saunders, a spokeswoman for the D.C. Department of Recreation. District revelers will count: 10-9-8-7-6-5-4-3-2-1-0.

"New York is wrong," she said.

Since Mayor Marion Barry started the city's festive New Year's Eve celebrations in 1983, the District has engaged in a spirited competition with New York. In 1984, D.C. officials invited New York Mayor Edward I. Koch to take part in the festivities, but he did not attend.

"It is going to be bigger and better," Jerri F. Williams, the coordinator of this year's celebration here said this week. A crowd of 150,000 -- about 50,000 more than last year -- is expected.

Highlighting this year's celebration on stages at 10th Street and Pennsylvania Avenue NW and 14th Street and Western Plaza, will be singer Stacy Lattisaw, a D.C. native, and performers Nona Hendryx and Roy Ayers. Other area participants include vocalists Willie Jolley and Gloria Hightower, keyboardist Dave Robbins' All-Stars and recording groups Maniquin and Stratus. Festivities begin at 9 p.m.

At 11:45 p.m., attention will turn to the stage directly in front of the Pavilion at the Old Post Office, where the midnight countdown will be led by Barry and 11-year-old Robin Williams, a sixth grader at Terrell Elementary School in Southeast.

Williams was a quarter-finalist on the television talent show, "Star Search," which aired Christmas Day. A lighted prototype of the U.S. Postal Service's 1988 Love Stamp will be lowered from the tower of the Old Post Office building and reach the ground at midnight.

Because of limited parking, D.C. officials suggest people use Metrorail, which will operate until 2 a.m.

Parts of Pennsylvania Avenue will be closed to traffic. Beginning at 10:30 a.m. on New Year's Eve, the northbound lanes of Pennsylvania Avenue between 10th and 11th streets will be closed. At 6 p.m., Pennsylvania Avenue between 10th and 14th streets NW will be closed. The avenue will reopen at 6 a.m. New Year's Day.

D.C. officials say no alcoholic beverages will be sold by licensed vendors outside the Pavilion. In 1986, the city's celebration was marred by drunkenness, injuries and a slaying.