What was going on when I was born?

IC Statements by U.S., page A5. A South Korean airliner missing with 269 people on a flight from New York to Seoul was shot down in the Sea of Japan by a Soviet jet fighter near a Soviet island off Siberia, the United States said yesterday. There were no known survivors of the attack, in which a heat-seeking missile was said to have been fired without warning at the airliner by an interceptor that had tracked it over Soviet territory for two and a half hours. President Reagan expressed ''revulsion'' over what he called ''a horrifying act of violence.'' He cut short his California vacation and called a National Security Council meeting in Washington today to discuss possible reprisals.

After nearly 24 hours of silence, the Soviet Union confirmed tonight only that its jet fighters in the Far East had intercepted and warned ''an unidentified plane'' intruding into Soviet airspace. But it made no mention of any attack on the plane. The statement, by the Government press agency Tass, was the first acknowledgment that Moscow knew about the South Korean airliner that Secretary of State George P. Shultz said was shot down off Sakhalin. (In Washington, the State Department said Foreign Minister Andrei A. Gromyko had sent a message to Mr. Shultz, along the lines of the Tass statement, saying that the plane had violated Soviet airspace, but seeming to disclaim responsibility for the downing of the craft. Mr. Gromyko added that search planes had found ''signs of a possible crash'' near Morenon Island, off Sakhalin.

BFMilitary Analysis Soviet sensitivity over foreign radar penetration of defenses guarding La Perouse Strait south of Sakhalin may have been the motivation for the reported destruction of a South Korean airliner by a jet fighter, according to American officers. In war the strait would be the direct route for the deployment of the Soviet Pacific Fleet into the northwest Pacific, the officers said. Over the last 27 months United States and Japanese intelligence services have reported the expansion of Soviet military power on Sakhalin, on the Siberian mainland and on the island of Etorofu of the Kuril island chain. Etorofu lies 150 miles northeast of Hokkaido, the northern Japanese island. The entire area, one officer said, is of major importance to Soviet strategic planning.

President Reagan today ordered a 2,000-member Marine force into the Mediterranean to position itself off Beirut so ''that all necessary measures'' could be taken to ''assure the safety'' of the 1,370 marines already on shore. Pentagon officials said the additional marine contingent would sail Friday from Mombasa, Kenya, and anchor off Lebanon in about a week. The officials said there were no plans now to send the additional marines ashore. The announcement of the President's decision was made in California, where Mr. Reagan is vacationing, by Larry Speakes, the White House spokesman. It came during a week when two marines were killed and 14 wounded in Beirut as leftist Moslem militiamen battled the Lebanese Army.

The Mesa Petroleum Company said yesterday that it had agreed to sell its stake in the Superior Oil Company back to Superior. The deal marks the third time in the last year and a half that Mesa has come away from a takeover opportunity without an acquisition but with a substantial profit. Mesa acquired about four million shares, or 3 percent of Superior, at an average cost of $34 a share earlier this year. Superior said it would buy the shares back at a price of $42 a share, yielding an indicated profit for Mesa of $32 million.

President Reagan expressed revulsion today at the reported downing of a South Korean passenger airplane off the Soviet coast. He declared angrily that the Russians had ''totally failed to explain how or why this tragedy has occurred.'' He also decided to cut short his vacation here by three days to return to the White House Friday for a National Security Council meeting to discuss responses to the incident. In addition, Mr. Reagan asked Republican and Democratic Congressional leaders to meet with him at the White House, possibly Saturday, to discuss the situation. ''I speak for all Americans and for the people everywhere who cherish civilized values in protesting the Soviet attack on an unarmed civilian passenger plane,'' Mr. Reagan said. ''Words can scarcely express our revulsion at this horrifying act of violence.

The disclosure statement mailed to the 80,000 creditors who have approved the Hyatt Corporation's plan to revive Braniff Airways contains a warning: ''There has never been an air carrier that has resumed an airline operation with as many as 30 aircraft following a reorganization under the Bankruptcy Code.'' The unprecedented nature of the attempt, approved yesterday by Federal Bankruptcy Judge John Flowers, is only one of numerous factors that make clear the extreme risk in trying to put Braniff back in the sky and keep it there, using the Dallas-Fort Worth Airport as its hub. Not only would a new Braniff need to offer fares and services that distinguish it from American and Delta, its two powerful rivals at Dallas-Fort Worth, but it also would have to break their stranglehold on the passenger traffic there. They control 80 percent of it.

On Wall Street these days, everybody seems to be on vacation, the weather is sticky and Labor Day, which normally signals revival, is three days away. But at the tables down at Harry's on Hanover Square, there is plenty of activity. ''It's a funny thing,'' Harry Poulakakos, the popular Wall Street restaurateur, remarked. ''Last year at this time, the market was going crazy and my luncheon business was quiet. Now, when the market's quiet, my business is good.''

A South Korean 747 was shot down in the Sea of Japan by a Soviet fighter near a Soviet island off Siberia, the United States announced. There were no known survivors aboard the plane carrying 269 people on a flight from New York to Seoul. American, Soviet and Japanese ships and planes fruitlessly searched the frigid seas north of Japan where the airliner apparently went down. There was an unconfirmed report that the long-missing plane had experienced radio difficulties and thus may have been unable to hear or respond to any Soviet warnings, but there was no clear explanation for the attack. (Page A1, Col. 6.) Moscow was silent for nearly 24 hours and then confirmed that its fighters had intercepted and warned ''an unidentified plane'' that it was intruding on Soviet air space. But the late report by the official press agency Tass made no mention of any attack on the South Korean airliner. (A1:3.)

''At 1838 hours, the Korean plane disappeared from the radar screen.

uf1 Friday VANYA' AT LA MAMA Andrei Serban, the innovative director whose Chekhov productions emphasize the playwright's comic intent, is presenting ''Uncle Vanya'' at LaMama Annex, 74A East Fourth Street, which has been covered in natural pine for the event. The set is on several levels, including a balcony, the audience sits on three sides, and the action occurs all over the place. The Rumanian-born director got his American start at LaMama, and almost everyone else involved in the production is an alumnus too. Joseph Chaikin plays Vanya. Santo Loquasto designed the set and Jennifer Tipton the lights. Mr. Serban doesn't believe in pregnant pauses in Chekhov, so the uncut production runs only an hour and 45 minutes, without intermission. Now in previews, the show opens Sept. 11. Tickets: $12 and $15. Reservations: 475-7710. FUJIBLOC ON 57TH ST.

The Marine Midland Bank announced yesterday that it was in the process of acquiring Carroll McEntee & McGinley Inc., a primary dealer in the Government securities market. Meanwhile, the Manufacturers Hanover Trust Company, the nation's fourth-largest banking organization, was quietly admitted to the prestigious ranks of primary dealers by the Federal Reserve Bank of New York. It had been the only major New York bank that did not hold this designation.