FOUR WAYS OF COMPUTING MIDNIGHT. By Francis Phelan. Atheneum. 212 pp. $15.95.

FORGIVE ME, I'm about to drop a name. I can't resist the temptation to mention Samarkand, Uzbekistan, which I recently and all too briefly visited. Samarkand, filled with magnificent 15th-century mosques and monuments and known as "the town of the 30 centuries," made me think of the first novel of Francis Phelan, one of 12,000 Roman Catholic priests who in the last 25 years have left their church. Four Ways of Computing Midnight, though it takes place mostly in mid- 20th century rural Indiana, concerns, as Samarkand does, the stars, the passage of time and the romance of the remote.

Its unnamed narrator begins life as an Irish immigrant streetcar man's son in North Pittsburgh, which he leaves, in 1944, when a vocation leads him to the Novitiate of the Holy Cross in Rolling Prairie, Indiana. At seminary, where he was "famous for being late; I never owned a watch and usually did not know what time it was," he gets appointed his class' Regulator, or timekeeper: in charge of waking his fellow students, summoning them to prayer, and keeping them mindful among other things of "the importance of midnight. So grave is the obligation to complete the office of the day that we anticipate, we say it ahead of time, so that we are never caught by the hour of midnight, which would be unthinkable."

They also learn the four different ways the church taught of determining when midnight comes: "Sidereal or Celestial time, . . . Standard time (arbitrarily divined up by man for his own use into time zones); . . . for us to calculate True Midnight, for example, we would have to subtract eleven minutes from the clock, which would of course be useless, for when the clocks in Rolling Prairie showed eleven minutes to twelve, True Midnight had already occurred!" What actually was being observed in Rolling Prairie, Indiana, and the rest of nation was, of course, War Time:

"Eisenhower and Patton had already freed Paris . . . Rome had been taken, too . . . Who would take Berlin?" Nobody questioned that "the world had entered upon a new age," or, after Hiroshima, that "man was in the final stages of human history." A visiting European priest, who had escaped a Nazi prison camp, is urged not to gloss over his account to the seminarians of what he witnessed there: "These men know what time of day it is," he is told. "It is unlikely anything you have to say will shock them. Proceed."

What he has to say, however, is profoundly shocking: the SS, he tells the students, "used people like meat . . . hung them up on hooks while they were still alive and their hearts still pounding." In particular, in the most affecting passages of all this novel, he tells of an SS victim named Mordecai who succeeded heroically, as he explained in his limited English, in his effort to "try to help my next" -- by which he meant his neighbor.

Phelan's narrator tries to help his own "next," a musical classmate who on Good Friday in 1945 did not merely sing the words of the Benedictus; it was as if his voice "was singing us all away, far away from the world; (as though his voice) had become the flickering tongue of flame, so that when it disappeared behind the altar I actually felt apprehension that it had gone out, that Billy himself had met his end," which in fact was what happened.

PHELAN BRINGS to life skillfully the griefs and rambunctiousness and the confusions of his theology students -- "if a man's business is below his belt," they are told, "he has no business, of course, being here" -- and the shifting moods of his own childhood when, bedazzled by a classmate, he fantasized running away with her "to fabled Samarkand (where) I defend her from all dangers . . . but I knew that I could never really go to Samarkand with her, or anyplace, if I was to become a priest."

Forsaking romantic notions involving girls and fighter aircraft, and overcoming a crisis of "the crowning sin of disbelief, of having doubted the existence of God," the narrator is ordained, his chalice having been designed by himself to incorporate his mother's wedding ring and his father's chrysoprase tie pin. He is assigned, among other places, to St. Elizabeths Hospital in Washington, D.C., where in one of the novel's many passages which may be based on truth he tries to convert Ezra Pound to Catholicism.

He puts behind him "the great music of civilization," the only thing he will really miss about religious life, and learns "without ever having been told, how to place the tongue, how to hold the throat," to make the Rebel Yell: "it comes to young men, certain young men, once every century or so, no one knows from where." He rebels, ultimately, because unlike many of his teachers he cannot profess what he has ceased to believe, and also because of a nun with analogous doubts, whom he keeps running into at seminars and in airports. This nun, whose name is Anne Francis, evokes the same feeling he had in grade school for the girl to whom he once shouted "a loud and respectful 'Hello' . . . the closest I ever came to saying what I felt."

In the early 1960s Anne Francis and the narrator both leave their orders and begin a new life together in a place more exotic than Samarkand: matrimony in what remains of the 20th century, determined always "to try to help my next," and to keep in mind the epitaph of an astronomer Phelan's father once knew: "I have loved the stars too fondly/ To have any fear of the night." Maybe similar thoughts comforted Ulug Beg, the martyred 15th-century Samarkand scientist, who also spent most of his life computing time.

Phelan, who won the 1982 Pushcart Prize for fiction, takes a big chance in this often elliptical novel, and sometimes asks too much of a reader's imagination, but he writes with such fresh and sensitive vigor that he whets our appetites, as few first novelists do, for a sequel.