July 8, 1983

President Reagan, ending protracted and intense speculation, said today that he would nominate Charles C. Cox to the Securities and Exchange Commission. Mr. Cox, 38 years old, has been the commission's chief economist since last September, when the post was created. He would replace John R. Evans, another Republican, whose second five-year term expired June 5.

Herman Kahn, one of the nation's leading thinkers on nuclear strategies and the future, died of a heart attack yesterday morning at his home in Chappaqua, N.Y. He was 61 years old. Colleagues at the Hudson Institute, a private center for research on national security and public policy, said Mr. Kahn, a prodigious worker and rapid-fire speaker, had undergone his annual physical examination last week and had been judged to be in excellent health. Mr. Kahn, who founded the institute in Croton-on-Hudson 22 years ago and served as its chairman and director of research, had been working in the last few days on a paper on nuclear warfare. He was to deliver it today at the Pentagon.

The Boston Symphony Orchestra under Seiji Ozawa and the soprano Leontyne Price will open the Berkshire Music Festival at Tanglewood in Lenox, Mass., tonight at 9 with a program featuring vocal works by Strauss and Brahms's Symphony No. 1. Brahms will also be performed at a prelude concert at 7 with Tom Krause, bass-baritone; Jules Eskin, cellist, and Gilbert Kalish, pianist. Sunday's program, starting at 2:30, will include Haydn's ''Creation.'' Tickets for tonight's music, including the prelude concert, are $9.50 to $45 for seats in the Shed or $7 for admission to the lawn; Sunday: $7.50 to $35 for the Shed, $5.50 for the lawn. Available at Ticketron outlets and through Chargit: 944-9300. Information: (413) 637-1666. FAIR DAYS IN WESTCHESTER A human cannonball, cow-milking contests, harness racing, a rodeo and elephant rides are some of the activities of the Greater Westchester County Fair taking place at Yonkers Raceway through July 17. The art of wool working, from shearing to weaving, will be demonstrated, and there will be rides, games, daredevil car stunts, livestock competitions and lots of food. Admission is $3 (free for under-8's; and today until 5 P.M., $1.50 for over-65's). The 98-acre site is 20 minutes from Manhattan. Information: (914) 968-4200.

The Aetna Life and Casualty Company agreed today to eliminate $203 million in tax benefits from its 1982 earnings, thus settling a dispute with the Securities and Exchange Commission over accounting practices. The restatement reduces Aetna's 1982 operating earnings by 38 percent, to $319 million, or $3.50 a share, from the $522 million, or $5.80 a share, originally reported. Revenue last year was $14.2 billion.

When a defendant, on the advice of his lawyer, confesses to a crime before the prosecution has begun its case, he has not been denied effective assistance of counsel, the New York Court of Appeals ruled yesterday. The court, the state's highest, held by a vote of 4 to 2 that a defendant's constitutional right to legal assistance could not be invoked until formal judicial proceedings had started. As a result, the court ruled, the confession could be allowed as evidence.