If a new skyscraper can't be great architecture, it can still make its mark as a dazzling presence. And if it can't be dazzling, it can at least be big.

The 975-foot Comcast Center is big.

The quicksilver obelisk, which will be formally dedicated Friday, elbows out the 945-foot One Liberty Place as the tallest building in the Philadelphia firmament. But since that history-making, tower is a mere 825 feet after you subtract its slender spire, the Comcast Center's arrival reconfigures the skyline more dramatically than the record books suggest.

Comcast, the tower, now rules the city's blue skies as completely as Comcast, the company, rules our blue screens. But beyond being top dog on the city skyline, and the 12th tallest building in America, what else can we glean about the rising media giant from the sleek contemporary design by Robert A.M. Stern and Graham Wyatt?

Not as much as we might wish.

Without a doubt, the Comcast Center at 17th Street and JFK Boulevard is a respectable work of architecture. The 58-story tower is clean in its lines, dignified in its stance on the grid, generous in its relationship to the city, responsible in its treatment of the environment.

Its developer, Liberty Property Trust, can boast - for the moment, anyway - that it is the tallest green building in the country, an achievement of no small import. Yet as the new headquarters for one of America's biggest conveyors of information, it is an oddly uncommunicative presence.

Despite a glass facade that sparkles from afar like a frozen mountain cataract, the tower appears anything but transparent. Its smooth, featureless face reveals little about its mighty occupant's ambitions, making it hard to get a fix on a company that is increasingly a fixture in our homes and palms. Comcast still knows more about us than we know about it.

If the new tower were simply another anonymous stack of office space, we might accept the inscrutability as an attempt to be all things to all people. While the project was envisioned in 2000 as a speculative building, it had clearly become Comcast's home by the time work started in 2005. The current design was custom-fitted for the company's coming-out as an information-age titan.

As Comcast chief Brian Roberts explained in an interview, he wanted a tower that was modern, but understated. So the planned Kasota limestone cladding was ditched in favor of fresher-looking glass, and the shaft was given a more tapered profile.

Roberts said he never cared whether the building was the tallest on the skyline, even though his $66 billion operation is Philadelphia's biggest corporation and will occupy 90 percent of the tower's 1.3 million square feet. "Fitting in has always been important for Comcast. We wanted to evolve the skyline, not change the skyline," he said.

Too bad he got what he asked for.

We live in an age ruled by media barons, a fiercely competitive moment when today's pacesetter is tomorrow's roadkill. The line between image and product has all but disappeared. By focusing on fitting in, Comcast missed a chance to use the power of architecture to convey information about its brand. Its abhorrence of showboating tells us only that this is a cautious bunch.

Robert Stern, ironically, knows a lot about branding. The dean of Yale's architecture school and a prolific historian, Stern became a big design name by making himself into the Ralph Lauren of architecture. Like Lauren, Stern clothes his clients in costumes that provide them with an instant pedigree.

Here, Stern wisely chose modern dress for Comcast, America's cable king. Yet his foray into modernism lacks conviction. There's no invention, just the assemblage of known ideas. Modernism becomes another costume.

Still, Comcast Center is a well-made one. It was a smart move to taper the tower. But a bolder architect would have tapered even more, especially toward the top, to form a sharper silhouette - and to make the building look less like a giant flash drive.

Glass now seems the obvious choice for the exterior because it offers abundant natural light. We're seeing a strong vitreous invasion in Philadelphia, a trend that began with the Cira Centre's rock crystal. But none of those take smoothness to the extreme that Stern does here.

Comcast Center's facades, clad in a heat-deflecting silvery glass, rise without any tactile projections from window frames or other elements. The high-gloss finish is meant to give the enormous tower an ethereal presence, like the Apple store cube on the plaza of Manhattan's GM Building. But at this scale, it just feels blank.

Our eyes have nothing to latch onto as they make the skyward climb. The only sense of scale comes from the cutouts on the north and south sides. On Arch Street, a towering wall of glass glowers at that relatively narrow street; it's nothing more than a vertical ice rink.

What saves the Comcast design are the corners. From the north and south, it appears as if Stern and Wyatt draped a pewter cassock over their obelisk. They create the effect by nipping pleats into building corners with super-clear Starphire glass manufactured by PPG in Pittsburgh.

The contrast gives the tower the dimension it desperately needs, and enables us to read the composition as an intriguing sequence of layers.

The layering starts at the base with the 120-foot-high Winter Garden, constructed from the same clear glass, and continues up through the crown. The clear glass emerges as if it were bursting free of its constraining silver garment, to become a diaphanous counterweight to the Winter Garden - a rather nice compositional touch.

Because the clear glass crowns the tower, it performs the mundane task of concealing the building's mechanical equipment. You can't see those boxes through the sheer veil, but a metal exoskeleton is easily visible. It gives the top a rough, industrial quality and, in the view of some, makes the tower appear unfinished.

I would argue that the sheer box brings the tower to a simple, logical conclusion, without the forced fussiness of a geometric hat or arbitrary spire. When the rectangular box is lit at night, it glows, putting the tower's recognizable stamp on the skyline. (Too bad the managers insist on that blue stripe.)

Down on the ground, along JFK Boulevard, is where Comcast Center excels.

The skyscraper is envisioned as part of a Rockefeller Center-type ensemble, situated between Suburban Station and a future building at 18th Street, so it sits back on a large plaza, designed by Olin Partnership. The plaza was born out of necessity because SEPTA tracks run below and it would have cost a bundle to build on top.

For a long time, the words urban plaza seemed to be code for barren wasteland. But here, the open space provides the huge tower with some breathing room and creates a gentle, dignified entry into the enormous volume of the transparent Winter Garden. Liberty's vice president John Gattuso worked hard to activate that plaza, and the underground mall that will extend SEPTA's concourse.

It's hard to talk about Comcast without mentioning the Cira Centre, the only other sizable office tower built here in the last 17 years. The petite Cira is more exhilarating, sculptural and satisfying as architecture, though it, too, has some blank moments.

But Comcast Center blows Cira away where it counts: at its intersection with the city. It's still early, but Comcast's plaza cafe and concourse mall promise to become a bustling urban nexus. How typically Philadelphian that the best feature of the city's tallest building should be hidden below ground.

Comcast Center, as much as Cira, was sited to take advantage of the city's great rail connections. Its Winter Garden is not just an airy lobby, it's intended to serve as Suburban Station's answer to 30th Street Station's great waiting room. Riding up the escalator from the concourse mall, I felt the same flutter in my heart that I do when I emerge from Amtrak's platforms at 30th Street.

The full-body immersion into the Winter Garden's drenching light is inspiring. But, as with so much else at Comcast Center, the thin architectural detailing left me deflated. An 87-foot-long, high-definition art video that is supposed to provide the missing aesthetic richness won't be activated until Friday.

Because Comcast Center is expected qualify as a green building, it promises both to spare our planet and offer its occupants a luxury cocoon of fresh air, rejuvenating natural light and high ceilings. It will use 13 percent less energy and 41 percent less water than a typical office building.

As with all green projects, the promised environmental benefits soon start to sound like the stuff of a TV infomercial. So, while Liberty Property gets points toward green certification for buying Pennsylvania-made glass, it's worth noting that the energy-efficient product must be shipped to Canada for framing, then trucked here. There go a few carbon credits.

Ultimately, though, it's the image of the great obelisk, shimmering like mercury in the afternoon sun, that many will remember. They may eventually forgive its vacuous facade, preferring to see the glass expanses as a blank canvas on which the city can project any dream it chooses.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.