What was going on when I was born?

Lebanon today formally canceled its troop withdrawal accord with Israel, declaring the American-sponsored agreement ''null and void.'' The Israeli Government, in a statement issued by Prime Minister Yitzhak Shamir's office, strongly condemned the Lebanese decision, describing it as ''a death sentence for Lebanese independence and sovereignty.'' The cancellation of the accord appeared to mark the end of the so-called ''Israeli era'' in Lebanese politics and to shift Lebanon solidly back into the Syrian-Arab fold. It signified a major victory for the Syrians and their Soviet backers, who have opposed the accord of last May 17 from the beginning because of the new relationship it would have established between Israel and Lebanon and because of the diplomatic achievement it signified for the United States.

Gary Hart's victory in the Maine caucuses Sunday propelled the Democratic Presidential campaign into an angry new phase yesterday. Walter F. Mondale acknowledged that he was fighting for his political survival and denounced Mr. Hart with the harshest personal attacks of the campaign year. Admitting that ''I got hurt bad'' by defeats in Maine and New Hampshire, Mr. Mondale for the first time centered his criticism on Mr. Hart's character and personality. In speeches in Boston, he depicted Mr. Hart as a coldly intellectual man who would bring an ''unfair, regressive approach'' to the problems of common people. Mr. Hart's strategists, convinced that the Mondale campaign was reeling, studied their private polls to see if the Senator from Colorado had a chance to deliver a decisive blow against former Vice President Mondale in the Southern primaries next Tuesday. A Hart adviser described the region as ''poised for movement'' against Mr. Mondale.

The National Can Corporation, the nation's third-largest metal can company, said yesterday that the investor Victor Posner, who now owns 37.2 percent of the company, had offered $40 a share for the rest of the shares, or a total of $410 million. A National Can spokesman said the company, which announced earlier that it was considering a leveraged buyout among other alternatives, is submitting the offer to its independent directors for evaluation. ''Right now, it appears to be a favorable opportunity,'' the spokesman said. She said the evaluation would take at least four months.

The Securities and Exchange Commission yesterday listed several prominent businessmen that it intends to call as witnesses in its case against Paul Thayer, the former Deputy Secretary of Defense. Mr. Thayer has been charged with helping eight individuals make $1.9 million by illegally providing them with insider information. The S.E.C., in a filing with the Federal District Court in Manhattan, also provided new details involving the commission's case. Included, for example, are names of the members of a backgammon group that the commission maintains traded information among themselves.. Among the 21 individuals the S.E.C. said it intends to call as witnesses are Harry J. Gray, chairman of the United Technologies Corporation; Thomas G. Pownall, chairman of the Martin Marietta Corporation; August Busch 3d, chairman of the Anheuser- Busch Companies; Jake Butcher, whose Tennessee banking empire collapsed last spring; Burt Lance, who was President Jimmy Carter's budget director, and Brynn Thayer, Mr. Thayer's daughter. None of these individuals has been charged by the Securities and Exchange Commission with violating any laws.

The nation's major automobile manufacturers reported today that sales rose 24.6 percent, on a daily rate basis, in late February. The companies, G.M., Ford, Chrysler, A.M.C., VW and Honda, sold a total of 248,062 units in the Feb. 21-29 period, which had eight selling days this year and seven last year. Sales for the entire month of February, with 25 selling days this year and 24 last year, were up 42.5 percent, to 655,016. The daily selling rate of 26,201 was the best daily rate for February since 1979. Last year the car makers sold 441,226 cars, or 18,384 a day, in the month.

BRUNSWICK, ME. JOHN PAPANIKOLAS was absorbed in an experiment in the organic chemistry laboratory at Bowdoin College the other day, taking about three-thousandths of an ounce of benzaldehyde and acetone, adding an aqueous ethanol solution of sodium hydroxide and pouring the mixture into a tiny flask. He watched the liquid drip through a filter, leaving a residue of yellow crystals that he patiently scraped with a miniature spatula into a test tube no longer than his little finger. Professors at Bowdoin believe that the sort of procedures being carried out in microscale by students like Mr. Papanikolas, an 18-year- old freshman from Tucson, Ariz., may change the way organic chemistry is taught to the nation's undergraduates.

''We are unable to perceive the Archbishop of Canterbury, the Vicar of Rome, or other powerful religious leaders behind every public acknowledgment of the religious heritage long officially recognized by the three constitutional branches of government.

The Supreme Court ruled today, 5 to 4, that a city may include a Nativity scene as part of an official Christmas display without violating the constitutionally required separation between church and state. The decision, which concerned a city- owned cr eche in Pawtucket, R.I., was the Supreme Court's most important ruling in some years on the permissible boundary between government and organized religion. It significantly shifted the boundary in favor of religion. While the Court has upheld numerous government programs that accommodate religious observance, including, most recently, state tax deductions for parochial school tuition and prayers by chaplains in state legislatures, the decision today marked the first time the Justices have permitted the official display of a symbol that is explicitly and exclusively Christian. Wide Impact Expected The ruling, Lynch v. Donnelly, is expected to have a considerable effect on official Christmas displays around the country. There has been uncertainty, and increasing litigation, for years over the constitutionality of officially sponsored Nativity scenes. The three-year-old suit over Pawtucket's cr eche, for example, prompted most Rhode Island communities to withdraw temporarily their own Christmas displays. (Page B7.)

SCIENTISTS using earthquake waves to chart the earth's interior, in much the same manner as the brain is scanned by computer-assisted tomography, have identified deep patterns that go a long way toward explaining the dynamics of continental drift, earthquakes, and volcanic eruptions. The wave patterns have indicated a deep- seated, superheated zone beneath the western region of North America, suggesting, it is proposed, that the western and eastern halves may eventually split apart. The three- dimensional mapping technique also appears to show that the former floor of the Pacific Ocean now lies under the East Coast and western Atlantic. These and other findings have led a number of specialists to conclude that the earth's mantle - the deep area between the rigid crust, or lithosphere, and an outer liquid core largely formed of molten iron - is not a single structure, as has long been assumed, but two zones that may have remained independent since the earth was formed. The scientists propose that earthquakes originating at a depth of about 400 miles in certain areas occur where slabs of sinking former sea floor bang against the bottom of the upper mantle and are kept from sinking farther. The absence of earthquakes farther down has been taken to mean the material is too soft for brittle fracture or that the region is relatively stable.

And 1984 promises to be nearly as strong for the makers of dishwashers, stoves, refrigerators, freezers, air-conditioners, microwave ovens and garbage disposals, according to industry analysts and executives. The industry shipped more than 32 million units last year, an increase of almost 6 million units over 1982. Sales exceeded $8.4 billion, according to the Association of Household Appliance Manufacturers, a Chicago-based trade association. ''It's the best year the industry's had since 1979, when we shipped over 33 million units,'' said Robert Holding, president of the association. ''Our 1984 forecast of 33.7 million units would make it the second best year in the industry's history.''

The Senate began a highly charged debate today on a proposed constitutional amendment that would reverse a 22- year-old Supreme Court decision banning officially sponsored prayer in the public schools. The debate was heavy with political overtones, since President Reagan has made the issue a staple of his campaign speeches and urged Congress to act swiftly. Senator Strom Thurmond, Republican of South Carolina, referred to the President's support in his opening remarks today and called the amendment ''of vital importance to the well- being of our nation.'' Senator Lowell P. Weicker Jr., the Connecticut Republican who is leading the opposition, accused Mr. Reagan of using the issue for political gain and said it was brought up ''to satisfy the President's conservative supporters.''

TUESDAY, MARCH 6, 1984 Companies Socal is to offer $80 a share for Gulf under an agreement announced by the two companies. If the bid is successful, it would be the largest takeover ever and would form the third-largest oil company. Atlantic Richfield, Socal's main competitor, dropped out of contention. (Page A1.) Antitrust lawyers predicted that a Gulf- Socal combination would win Government approval, pending divestitures and other steps to assure that competition was not significantly diminished. (D5.) Gulf, after a long, steady decline in its oil and gas reserves, had come to be viewed on Wall Street as the most likely merger candidate among the major oil companies. But experts are astonished that a battle begun by the chairman of Mesa, T. Boone Pickens, could lead to the acquisition by Socal. (D4.) National Can said Victor Posner, the Miami investor who owns 37.2 percent of the company, had offered $40 a share for the rest of the stock in the nation's third-largest metal can company. The total deal was valued at $410 million. (D1.)