In the somber gray light of an overcast morning, four hours before the museum opens its doors, a dozen or so staffers work quickly to assemble the original dawning of the National Gallery East Building's well-known sky.

The set pieces of this meticulous theater come out one by one. Some are fastened together at the ground level, some on the mezzanine floor, with a precise system of counterweights.

It's a child's game of hooks and hoops, of winged invaders and sailboat rudders -- that is if it weren't for the serious demands of the process and the magnitude of the spectacle, gradually branching outward.

"This is a monumental task," Shelly Sturman, head of object conservation at the National Gallery of Art, says of the long-awaited return of the late Alexander Calder's untitled mobile. It was taken down April 19, 2004, for restoration. With a new paint job and "hardfacing" work (a surface treatment that will allow the pieces to move more freely on their connecting loops), the mobile should behave more like a mobile these days.

"This time we gave it the full treatment," says Sturman, who also worked on the museum's 1988 restoration -- the only other time the sculpture had been taken down since it debuted with the angular East Building in 1977. Twenty minutes before the museum opens, the 76-foot, 920-pound aluminum structure gets its final lift. The staff, perched silently at various corners of the atrium, sees the Calder ascend the final inches and suddenly begins to clap and whistle.

Of course, the floating dream work of the mobile -- the form most associated with Calder's good-natured genius -- has been co-opted by makers of crib ornaments and, in recent decades, has won a place in American homes by silencing wide-eyed newborns with its fluttering guile.

When given a mobile to call their own, adults usually don't fare much better. Susana Arellano stands spellbound on the upper mezzanine for nearly 15 minutes, gazing eye-level at the sculpture.

A reporter eventually taps her on the shoulder, though it seems a shame to do so. She isn't startled.

"No, I feel very relaxed," says Arellano, a 22-year-old architecture student from Mexico City who timed her vacation to see the work for the first time. "I was just thinking that, when you're a child, you have so many fantasies. And art like this, for me, is like making your fantasies real."

Susan Malbasa of Sonoma, Calif., is on the lower mezzanine. She places her hands on her hips in a slightly less serene posture. "I just want to grab it and spin it around," she says.

It isn't an impossible assignment, since the lowest wing of the mobile can hover just seven feet from the mezzanine floor at times, but it's suggested to her that some childish fantasies are better left undisturbed.

"Come on," she says, "it's a mobile -- don't you want to spin it?"

"Sandy [Calder] was always making delightful objects for the childlike parts in all of us," says Paul Matisse, grandson of Henri. Calder asked Paul to fabricate his sculpture and Matisse was on hand yesterday to direct much of the reinstallation. "His mobiles were an extension of his delight. Notice how the shapes are almost haphazard, not serious at all."

There are, of course, many other things to love about Calder's mobile: the tenacious agreement between material chaos and gravitational order, the simple free-form of color (the orange is called Calder Red), the splash it creates against the cooler angles of architect I.M. Pei's fortifications.

But it's the new discoveries, Matisse says, that he enjoys most. He recalls a time when Calder evaluated an early working model of the mobile.

"He asked, 'Why aren't they all flat?' " Matisse and his team had made the hanging structures on the mobile slightly off kilter, as though the planes were soaring above the ground. But Calder, Matisse says, wanted the mobile to convey a sense of balance in all of its elements, not just in the practical goal of preventing the sculpture from crashing down to one side. "And now it makes great sense to me why he wanted the pieces to be flat, even though many thought it was more exciting the other way. It's better because now it's exactly balanced. That deeper vision of balance is really something to behold, and something that we would all like the world to have."

Calder "was always making delightful objects for the childlike parts in all of us," says Paul Matisse, artist and engineer, who directed the assembly.Above, the Calder mobile is reinstalled by Michael Belman, left, Andrew Krieger, Dan Shay and Abigail Mack. At right, museum staffers Carol Bridges, center, and Stephanie Wright, right, applaud. "This is a monumental task," says Shelly Sturman, left, who heads object conservation at the gallery.