June 3, 1986

ASTRONOMERS studying one of the most ancient mysteries of science, the origin of the moon, are embracing a theory that just a few years ago seemed fantastic: that the moon was born in a catastrophic collision between the earth and another planet the size of Mars. The hope of establishing the moon's origin was a primary scientific rationale for the manned landings of the Apollo project in the 1960's - the great question that Apollo failed to answer. Three theories competed for respectability as the landings began, but to the bewilderment of astronomers, the long-awaited flow of chemical and seismological data seemed to support none of the three. Now, encouraged by new computer simulations, a growing consensus among astronomers favors a fourth idea, the ''giant impact'' or ''mega-impact'' theory. It was ignored or derided by most who heard it when it was first put forward, in 1975.

Canada today retaliated against a United States tariff on cedar shingles with a barrage of tariffs on American goods, contributing to mounting trade tensions between the world's two biggest trading partners. The Canadian Government restored previously abandoned tariffs on some American books, periodicals, semiconductors and computer parts. On all the items, Canada had earlier voluntarily lowered tariffs below levels set by the General Agreement on Trade and Tariffs. On a number of other items, Canada raised tariffs that were below levels permitted by GATT. These included tea and teabags; oatmeal and rolled oats; certain trees, including Christmas trees; cider, asphalt and air filters.

''I have not the slightest desire to return. I think anyone in a sound mental state would not want to return from freedom to prison.

The New York City Police Department will begin mandatory drug testing of all members of its organized-crime control bureau, the unit that includes the narcotics division, under an order that took effect yesterday. The order marks the department's first effort to require all members of a particular unit to submit to periodic drug tests without advance notice. Chief of Personnel Richard J. Koehler said the department hoped to test each officer in the bureau at least once a year.

A brief report on May 21 about daylight time around the world misstated the practice in Israel. That country, like many others, moves its clocks ahead one hour. The practice began this year on May 17, approved by Parliament over opposition from some Orthodox religious groups.