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James Turrell is speaking to a group of students and admirers in the grand old auditorium of Cooper Union's Great Hall in Lower Manhattan. The subject of tonight's lecture is an endless source of fascination for him: light and space. For those who like their art gurus to look the part, Turrell does not disappoint. After many years in the desert, his skin has acquired a leathery sheen. He has a long white beard, white hair, and the piercing blue eyes of a nineteenth-century romantic visionary. Bounding across the stage, carrying his tall frame with a formidable presence that's a bit intimidating, Turrell speaks in the clear, unaffected manner of his Quaker ancestors. He jokes to the crowd that in the time it has taken for him to complete his life's work--the Roden Crater Project, a series of "light observatories" carved into an extinct volcano in Arizona's Painted Desert--his daughter "was born, grew up, went through the various levels of schooling, attended medical school, and became a doctor." Laughter. "And I'm still not finished!" More laughter.

His humor acts as a counterweight to the seriousness of the project, which has been referred to as the Sistine Chapel of the United States. It's certainly a project that the art world has been talking about for decades, in large part because no one really understands exactly how it will function once it's completed. Roden is both a work of art, and a collection of built spaces designed for the observation of light.

Originally linked with the California-based "light-and-space" art movement of the 1970s, Turrell still occasionally shows light-inspired art pieces in galleries (a retrospective of his work is scheduled for this fall at SITE in Santa Fe, New Mexico), but his most ambitious work right now is architectural. Modeled after ancient naked-eye observatories, the design for Roden Crater consists of more than 1,000 feet of tunnels leading to seven viewing chambers, each one built into the side of the crater. An additional space called "The Eye of the Crater" will be located 38 feet below the floor of the Roden bowl. The skyspaces (as Turrell refers to them) look out through classical archways and are situated around the huge crater at different elevations. One of the spaces will operate like a pinhole camera, projecting images of both the moon and sun onto a stone slab in the center of the room. Another will isolate the light of stars more than one billion years old.

The Crater is ideally suited for observing the sky be-cause it's located in an area with incredibly low light pollution. "When there's an event in light on the horizon or in the skies, there's an event in light in the Roden spaces that I am creating," Turrell says proudly. "I create a world that deals with how we perceive, so that you look at yourself looking. At the same time there is a physicality to the light that makes it a material." Near the top of Roden is an open-air observatory that will enable visitors to experience "celestial vaulting," an effect created by framing the sky so that it appears close enough to touch. When the crater opens to the public next year, only fourteen people a day will be allowed in: six day-trippers and eight over-night guests, who will sleep in spartan quarters furnished with spare Turrell-designed tables, chairs, and beds.

Turrell can rhapsodize for hours about the quality of light at Roden Crater. "It's possible to mix this light that is older than our solar system," he says, referring to the light of old stars that will be visible from a skyspace on the north side of the crater, "and have it bottled in a space so that you feel it physically present. You can touch it." Turrell then goes on to talk about the contrast between old light and sunlight, which is just minutes old when it reaches earth. "You can have new light, eight and a half minutes old, and light that is billions of years old, and you can actually be in close contact with both of them."

When talking about Roden, Turrell becomes so excited that he often, by his own admission, loses grasp of reality. "The megalomania of artists!" he says. "You draw these things out on paper and then you do some chewing at the top and you realize you are moving 800,000 cubic yards of earth just to make this thing the right shape. And you've hardly changed the damned thing at all!" A few years ago he was included in an exposé on the Dia Foundation in Vanity Fair, which characterized him as an artist whose financial demands nearly bankrupted the arts organization. Today he's reluctant to answer questions about the cost of the project (reportedly it's $22 million).

Turrell tells the Cooper Union audience that when construction finally began at Roden two years ago he wasn't even aware that he needed a building permit. "I told the local planning board: eThis is art.' They looked at the plans and said, eIt looks an awful lot like a building to us,'" Turrell says, eliciting more laughter from the crowd. "In fact, at 600 feet, the Crater is one of the tallest buildings in the state of Arizona." With the help of local supporters, Turrell marshaled enough evidence to show that Roden Crater will be similar in purpose to a Navajo kiva, which is exempt from building codes due to its ceremonial nature. Later that same year Turrell managed to convince Coconino County's building department to pass a bill designating a new category of building: land art. Sitting in a roomful of future architects, I wondered how many of them would ever wield the same kind of power over local planning boards.

Coconino County is not alone in its confusion over the nature of Turrell's project. Is it art or architecture? For almost 30 years Turell's work has enigmatically straddled the two disciplines. Writing 10 years ago in the New Yorker, Adam Gopnik anticipated this development in the artist's career when he observed that Turrell seems to have more in common with California architects like Richard Neutra, whose "picture window-filled houses attempt to achieve an equilibrium between an enveloping light and serene interior." In fact, a number of important architects cite Turrell as a major influence: Michael Gabellini, Steven Holl, Billie Tsien, and Tod Williams, for example. And although he was never formally trained as an architect, Turrell seems to have an uncanny, intuitive grasp of the discipline. "James is able to give me complex verbal instructions over the phone with complete dimensions and coordinates that translate into finished drawings and models," says Janet Cross, the New York architect who created the models and drawings for Roden.

Earlier this year Turrell's light tunnel at The Museum of Fine Arts, Houston was completed. The passage connects the museum's 1957 Mies van der Rohe addition with the recently opened Rafael Moneo addition. "These light tunnels are very difficult," explains Turrell, who has also designed a similar one in the Netherlands. "They're basically dead spaces." The Houston project combines a "Ganzfield space" with "shallow space" construction to create a unique experience in light. "Ganzfield" is a psychological term referring to the perception of limitless scope. It's achieved here by using a single source of illumination in the middle of the tunnel. Shallow-space construction is created by blocking off the source of light to create an illusion of two-dimensional space. In Houston, visitors experience an abrupt change in perception toward the end of the tunnel, when the light suddenly becomes two-dimensional.

Turrell's plan for the Friends Meeting House, also in Houston, will be completed later this year. In his design for the Quaker structure, Turrell took Meeting, the light project that he installed in 1986 at P.S. 1 in Long Island City, New York, and turned it into a building. Meeting is a trapezoidal chamber with a rectangular opening cut into the ceiling so that the sky becomes the dominant feature. Tall, sloping plywood benches are arranged along the base of the room. When Billie Tsien was teaching architecture at Yale, she took her graduate students to see it. "One of the things that he does is make spaces about emotion," Tsien says. "At P.S. 1, you walk into the room without any expectations. Then you realize there's no roof, and suddenly you feel something that is emotional rather than intellectual. It is about body experience, more visceral than intellectual. The feeling is calming and freeing."

Although Roden remains Turrell's main obsession, he says that he would like to work on more building commissions and "autonomous structures," follies with no clear function. Re-cently he and Tadao Ando designed a combination hotel and art gallery on Benesse Island in Japan. It included an elliptical rim and a Turrell skyspace. For the project, Turrell met with Ando and sketched out the building's shape and skyspace; Ando completed the rest of the building's design.

Turrell's forays into architecture aren't always so harmonious. In March his design for "the House of Light," in Santa Fe, was rejected by the clients, Michael and Jeanne Klein. Turrell and Cross had come up with an elaborate all-glass pavilion designed to take in the 360 degree view that the site enjoys from a ridge in the Sangre de Cristo Mountains. The house would have contained many of Turrell's light projects. Turrell designed a bathroom shower that would issue beams of colored light from the showerhead when the water was turned on. The garage was designed as a Ganzfield space; a curb would stop the car from crashing into the back wall. The Kleins seemed like ideal clients for Turrell. Michael Klein described himself to me as an avid art collector who thinks about God every day. After canceling the Turrell project, he refused to talk about the decision, but in an earlier interview he talked about Turrell's genius and referred to him as "a great humanist."

The Kleins own several Turrell light pieces. The commission came about when they told the artist that they were building a house, and Turrell asked if he could design it. "I said, eWhat do you know about architecture?'" Michael Klein recalled at the time, "and he told me that he had been designing houses. We've had to reign him in several times. He would like to turn every room into a light piece. I don't want to be disoriented in my own house. Nor do I want my guests to be." "You can push a client so far, and I pushed him too far," Turrell says of the Kleins' decision.

"You have this problem with architecture in that people want to use the building. They may not be interested in the issues that the architect is involved with. Architecture has a lot to do with understanding what people want to experience." Turrell is used to people having difficulty with his work. His tunnel in the Netherlands included vapory light that caused people to become disoriented. In the early 1980s a visitor to the Whitney Museum of American Art sued him when she fell over after attempting to lean on what turned out to be a wall of light. "Some people love a roller coaster," Turrell responded glibly when asked about the lawsuit. "There are others who, if they weren't planning to go on one, certainly didn't want to get on it."

In one way or another, Turrell has always been working with architecture in provocative ways. Born in 1943 in Los Angeles and raised in Pasadena, his early influences were not the Arts and Crafts houses of his suburban neighborhood, but Quaker meetings and air flight. His father, Archibald, was an aeronautical engineer who taught at a local technical school and collected vintage airplanes. Turrell attended Quaker meetings with his family and learned to fly. He majored in psychology at Pomona College in nearby Claremont, California. To support himself through school, he woke up before dawn and flew the milk runs into the San Joaquin Valley. "In the valley, there's a low-lying morning fog that hovers about six feet off the ground," Turrell says. "As the sun came up incredible things would happen to that fog."

Following his college graduation in 1965, Turrell was involved in a CIA-sponsored initiative to fly Buddhist monks out of Tibet, but he refuses to talk about that phase of his life. At the Cooper Union lecture, he confesses that he once applied for graduate school there and was rejected. In the late 1960s and early 1970s, he studied sculpture at the University of California, Irvine, and at The Claremont Colleges. For one of his first art projects, he took a couple of rooms in the old Mendota Hotel in Santa Monica and, blocking off the doors and windows, manipulated beams of light through a series of slits in the wall. One room contained light captured from a traffic light. Turrell's art took ordinary objects--a light bulb, a skylight, a frameless window--and turned them into ecstatic experiences.

For Turrell, art and architecture and light and mysticism are one and the same--and they all come together in flight. He refers to the sky as his big studio. "The fact that new space is disorienting is something that we've certainly experienced in flight," Turrell says. "Perception is difficult in the sky. The work that I am doing will change how we orient ourselves to spaces. We will be able to negotiate spaces that were more difficult than before. We do that in flight, and we do it in architecture."

Turrell discovered Roden Crater in the early 1970s while flying over the southwest as an aerial cartographer, his day job then. Swept up by the land-art movement of the time, Turrell had set out to look for an extinct volcano. He wanted one that would resemble the truncated pyramids of the ancient sites that he had been studying. He found three. "Thank God, I was able to get Roden," Turrell says. "There were two others that I thought this was possible in. One was owned by the Santa Fe railroad. The other is four times the size of Roden, and so that would have been 16 times the cost. It's lucky I didn't get that one."

Initial funding for Roden came from a Guggenheim grant and the Dia Foundation for the Arts. In the mid 1980s, Dia collapsed and Turrell was forced to look for other funding. (Dia has since been restructured and re-sumed operations.) He has received grants from the MacArthur Foundation and Count Giuseppe Panza di Biumo, an Italian art collector. Four years ago the project was given new life when funds from the Santa Fe-based Lannan Foundation were used to set up the Skystone Foundation, which now oversees construction from an office in Flagstaff. "It was probably good that I didn't have the money to do it at first," says Turrell, "because I got to think about it more. Maybe what moves art into architecture is having to consider more things."

Turrell lives on a cattle ranch that's within a half-hour drive of Roden Crater. Over the phone late at night from the ranch he sounds like one of those sleepy truckers that you may have heard droning away on CB radios in the Southwest. He's just returned from Japan, where he's designing a building, and about to go to the South of France, where he is working on the lighting for the Pont du Gard, a Roman aqueduct. I ask him to tell me a little bit more about his criteria for choosing the Roden site. "I wanted a crater that went up at least 200 feet, so that you'd have the dollhouse effect when you look down below," he says. "At 600 feet, with the 360-degree views, it's incredible. Especially in the summer when you get these puffy cumulus clouds that are flat on the bottom and have the rain that hangs down like tentacles from a Portuguese man-of-war."

Turrell talks me through the design of the crater. "As you go up to the top of the crater," he says, "you will have to go through this tunnel, which takes you through a progression of spaces--through the fumarole space, and the sun and moon space. The reason for that is to help you to adapt to the dark. You will come from desert sunlight, which is extraordinarily bright, then you go into these darker spaces. You know the feeling of coming out of a movie theater during the day into this brutal light? This way you'll transition in a series of spaces so your dark adaptation is erased in a reasonable manner."

As he began doing 30 years ago in the Santa Monica hotel, Turrell is controlling the source of light in each of the skyspaces and in the observatory on the crater's top. From what I can see in photographs of the skyspace models, the structures look like bunkers that are built into the side of the crater, and approached through a series of doors and steps. But that's not exactly what they are. "What looks like a door is actually an aperture," Turrell says. "What looks like stairs are in fact baffles for light. What looks like a normal room is an inside space made to accept light, a sort of camera. On the floor I will put black, volcanic sand that's anti-reflective."

I ask Turrell what connects his art to architecture. Why are his studies in light showing up in minimalist buildings all over the world? Why is he known, in some quarters, as one of the most influential architects working today? "In the last 20 years there have been some very influential non-building architects," he explains, "I am one that actually likes to see his structures built. I am doing an architecture of space and light. A topological architecture. That doesn't mean I don't have to deal with enclosure or with form, but I want to make most prominent that which is between, as opposed to that which makes the between. It's very simple architecture. An architecture of light."

Jeffrey Hogrefe writes extensively about art, design, and architecture. He lives in Brooklyn.