Mikhail Gorbachev is back -- looking tan and healthy. We still don't know where he was for seven weeks, but his absence from public view reminds us of how much we do not know and often cannot find out about the great power with which we so often deal on matters of supreme importance.

Gorbachev's breezy personal style and public relations skill, his campaign for ''glasnost'' and ''perestroika,'' invited us to forget that the men of the Kremlin control the flow of information as tightly as the flow of emigration.

But non-events like Gorbachev's non-appearance are a sharp reminder that the politics of the Soviet Union remain almost wholly hidden from view. Bits and pieces of information are studied by Kremlinologists with the care soothsayers once devoted to the entrails of goats. Rumor and speculation supplement information.

Thus a well-connected West German newspaper, Bild, broke a story on Gorbachev's absence with reports of rumors that, after three weeks of vacationing at Yalta, Gorbachev had become very ill with food poisoning and after a certain number of days had returned to Moscow by train, where he was met by six black limousines, one of which was in fact an ambulance.

Bild also noted that Gorbachev's departure for his vacation had not been announced in advance, as is usual, but only when, near the end of August, it was necessary to explain the First Secretary's unavailability to a group of visiting U.S. congressmen.

Bild speculated that the poisoning may have been deliberate, but noted that the Soviet trade union paper Trud featured a story on the many illnesses in the Soviet Union caused by poor sanitary conditions.

Meanwhile in Eastern Europe, speculation grew about a political struggle in the Kremlin that pitted Gorbachev against his deputy, party ideologist Yegor Ligachev, whose public appearances increased during Gorbachev's absence.

The whole episode reminded seasoned Kremlin-watchers of other times when illnesses and struggles have been hidden from public view.

Harvard professor Adam Ulam wrote of Lenin's last illness in ''The Bolsheviks,'' ''It was the Politburo over and above his doctors that regulated Lenin's treatment . . . the patient and his family could but hopelessly witness how political rather than medical considerations were applied in ordering his complete isolation from the outside world.'' Lenin complained that he was ''not a free man'' and bitterly protested harassment of his wife by party rivals -- a fact quickly recalled when Raisa Gorbachev failed to appear at a scheduled meeting.

While Lenin lay dying, the world wondered. It was not the last time isolation and secrecy shrouded a Soviet ruler. Leonid Brezhnev was dead for several days before news of his death was released. And Yuri Andropov's fatal illness was concealed for weeks.

We know little more about the politics of the Politburo than about the health of its members. Lavrenti P. Beria was considered a major power to the very day of his downfall and arrest. Soviet specialists in Western intelligence agencies were as surprised as Nikita Khrushchev himself by Khrushchev's fall from power.

Even today Kremlinologists are uncertain about what happened. They believe Khrushchev had real enemies in the military establishment -- enemies who were probably aided by the KGB. So, of course, does Mikhail Gorbachev. Khrushchev was charged with ''subjectivism and spontaneity,'' with making ''individual decisions'' and with ''violating the principles of collective leadership,'' which, loosely translated, means alienating colleagues who eventually dismissed him.

There are striking parallels between the Soviet Union's most vigorous charismatic leaders -- Khrushchev and Gorbachev. Both understood that telling the truth about the Soviet past is necessary to invigorating the Soviet present; that Soviet bureaucracy has long since become an obstacle to Soviet development; and that the party has become part of the problem.

Whether these shared characteristics will lead Gorbachev to the same end as Khrushchev, we do not know. We will have to wait, remembering while we make momentous decisions -- on the INF Treaty or the deployment of SDI -- that verification of the whereabouts of anything is very difficult in a closed society.