For millennia, a comet appearing in the night sky was considered an omen of cataclysmic events, a heavenly harbinger of the fall of empires, shattering earthquakes, the demise of kings. For Tin Hat Trio, comets have come to represent disasters of a personal nature.

The uncategorizable acoustic ensemble, which came together in San Francisco in the mid-'90s, marks the release of its fourth album, "Book of Silk" (Ropeadope Records), on Tuesday. The trio -- accordionist-pianist Rob Burger, violinist Carla Kihlstedt and guitarist Mark Orton -- offered a preview of its moody, often unsettlingly beautiful new music in June at the opening performance of Stern Grove's free summer concert series.

Since its first release, 1999's "Memory Is an Elephant" (Angel), the trio has drawn on a far-flung array of influences, from post-bop jazz, tango and blues to contemporary classical music, Eastern European folk traditions and the film scores of Nino Rota. Tin Hat has added a raw, emotionally charged edge to its sweeping cinematic sound, joined on "Book of Silk" and at recent shows by Zeena Parkins, the harp virtuoso known for her work with Bjork and as a fixture in New York's avant-garde music scene, and tuba master Bryan Smith, a player equally at home performing in symphony orchestras or with his band Deep Banana Blackout.

The album is titled after an ancient Chinese manuscript (circa 400 B.C.) that contains the first definitive atlas of comets, according to archaeologists. The book includes about two dozen renderings of the elusive celestial bodies and, in some cases, the volume's pages roll out more than a yard. Below each picture, a small caption describes a calamitous event that the comet's appearance seemed to anticipate, such as "the death of the prince, " "the coming of the plague" and "the three-year drought."

With its catalog of devastating portents, "Book of Silk" became sadly apt as an album title in the wake of the sudden death last year of Orton's wife. Singer-songwriter and artist Lauren Orton, 32, drowned in April 2003 while river rafting in Washington. The trio recorded the album only months after the accident.

"After Lauren's death, it shatters your belief system, and you're trying to pick up the pieces," Orton says. "That's why that silly term is out there. You're trying to reorder your universe, trying to make sense of things, and I think that's why it resonated with me, seeing these people trying to explain these inexplicable events, trying to tie the comets to events on Earth."

While much of the album's music wasn't composed directly in response to the tragedy, Lauren Orton's presence often can be sensed. Orton wrote the melancholic opening track, "The Longest Night," in the weeks before her death, and would often play it at home as a piano solo. "It was driving her crazy," Orton says. "She was saying, 'You're not allowed to play that tune while I'm in the house. It's way too sad.' Whether or not it was foreshadowing, it ended up functioning that way."

The evolution of Tin Hat's sound is a study in the mysterious process of creative osmosis. On 2000's "Helium" (Angel), the group expanded its growing list of instruments to include prepared piano, dobro, banjo and marxophone, and enlisted the services of singer Tom Waits on the title track. And on 2002's "The Rodeo Eroded" (Ropeadope), the three musicians dived deep into the heartland, immersing themselves in blues, bluegrass and old-time country. The trio's Americana credentials were cemented when Willie Nelson crooned a haunting rendition of the standard "Willow Weep for Me." The percussion contributions of Phish's Jon Fishman and Medeski, Martin & Wood's Billy Martin, on everything from quica (Brazilian drum) to fire extinguisher, added considerably to the group's already inventive rhythmic palette.

The remarkable empathy evident in the trio's playing is rooted in a long personal history dating back to school days. Shortly after the three relocated to the Bay Area in 1996, they founded Tin Hat as a composers' collective, committed to developing a volatile acoustic music that blends composition and improvisation. Sometimes it seems the more Tin Hat Trio's members work apart, the better they sound together.

Orton, who now lives in Portland, Ore., has been working as a film composer, gaining recognition with his score for Miguel Arteta's "The Good Girl" starring Jennifer Aniston. Oakland-based Kihlstedt has spent much of the past two years on the road with Sleepytime Gorilla Museum, the art rock band created by former Idiot Flesh provocateurs Nils Frykdahl and Dan Rathbun, and her own band, Two Foot Yard, which released an eponymous album of songs on John Zorn's Tzadik label last year. Burger, who now lives in Brooklyn, has become an arranger for prolific producer Hal Wilner, writing charts for projects focusing on the music of Nino Rota and Leonard Cohen, the latter featuring vocalists Rufus Wainwright and Nick Cave.

The biggest change for Tin Hat is its expansion from a trio to a working quintet. Harpist Parkins and tuba player Smith contributed to "Rodeo Eroded," but their roles have expanded greatly on "Book of Silk," meshing with Tin Hat's avant chamber aesthetic.

"They're both players who cross over between jazz and improvisation and classical music," Orton says. "They're playing instruments, tuba and harp, that are sort of idiomatic, even more than guitar, violin and accordion, but they're using extended techniques and playing their instruments in ways that are really original. We're still a chamber group, but we have even more flavors in it now, and at times it ends up sounding like a miniature orchestra. "