It is midnight in Rome, and the massive dome of St. Peters's Basilica looking up silently in the dark, catching the summer moonlight that baths the marble staues atop Bernini's coloanado.

The windows of the Renaissance polozzo where Pope Paul VI has his apartment are unlit, and the hordes of tourists who visit during the day, are gone.

At night St. Peters's Square becomes an oasis of quiet and repose, worlds away from the frenetic Roman summer night.

Vatican City is generally quiet these days not, altogether surprising since the pope has left Rome for his summer palace at nearby Caselgandolfo. He will remain there until mid-September returning only days before his 81st birthday.

With the pope away, most of the top officials of the Curia, the Vatican's governing body, have also left town and only a skeleton force of top prelates had remained the administer the smallest city-state in the world.

The prevailing mood is perhaps fitting for what all agree is the twilight of Paul's papacy, a period of waiting in which little is expected to happen and most minds are turned toward the succession.

AT THE END of his 80th year, the pope is lucid and alert, but time has taken its toll. Wracked by a painful form of arthritis that has left him virtually crippled, the pontiff has severely curtailed his activities.

In the last year he has cut his audiences by half, and the only recent occasions on which he has left the Vatican were his trip to Castelgandolfo and two months earlier, the state funeral for Aldo Moro, the former Italian premier who was, before his murder by leftist terrorists, the pope's long-time friend and protege.

In recent years, the pope often has spoken publicly of death. Some Vatican insiders, however, describe his health as a "bad iron constitution" that could keep him alive past the time two years from now when the Vatican pecking order will be thrown into chaos by the required resignation, at 75, of the secretary of state, Jean Cardinal Villot.

Although he has recently spoken out forcefully on issues like abortion and the Soviet dissident trials and has received foreign visitors, the pope's failing health is reportedly limiting his decision-making ability.

"There is a feeling of a lack of leadership," a highly placed prelate said the other day. Since Vatican strongman, Msgr. Giovanni Benelli, became a cardinal last year and moved to Florence, "No one really knows who is in charge," he added.