Even if you don't believe in astrology, you'd have to agree that the heavens have guided Dava Sobel's fate. Astronomy has brought her fame and fortune—once she finally found her polestar in the early '70s.

She was working in public relations for Ithaca College when she heard Carl Sagan give a talk "about the possibility of discovering other solar systems.... And I was so fascinated. I remember feeling, this is a life-changing moment. I just want to learn about these things, I want to write about them. This is what I'm going to do now."

Fortuitously, Sobel was asked by the student editors of an underground paper (this was the early '70s, after all) to interview Sagan for the grand fee of $5. The real reward, of course, was getting to know the celebrated astronomer, who taught at Cornell and recommended her for a science writer's position there.

Three decades later, Sobel is still looking to the heavens and still writing. Viking is launching her latest, The Planets,with a 300,000-copy first printing and a one-day laydown on October 11. Later in the month, Walker & Co. will bring out a 10th anniversary hardcover edition of the book that launched Sobel's career: Longitude,the now classic tale of a provincial British carpenter turned clockmaker who solved the overarching scientific problem of the 18th century: how to find longitude at sea.

Despite what the heavens may predict, success is never inevitable. In retrospect, Longitude's dramatic story of quest and triumph, its underdog hero and Sobel's artful yet effortless writinglook destined to have been a bestseller.

In fact, she says, Longitudewas turned down by 12 British publishers ("Brits don't like to get their history from America," she was told) before Fourth Estate took it up. The skeptics got it wrong—the book was an even bigger hit in England than in the U.S.

After Longitude's success, Sobel returned to a project that had fallen through earlier, an illustrated book about the planets. She reconceived the book with text only, since astronomical photographs are readily available on the Internet.

"It really had to be a different book. I walked around for a year just thinking of how to do it because so many books had been written about the planets—what could I contribute that would be different?"

And then Sobel's agent, Michael Carlisle, unwittingly provided her with the answer. "He's somebody who is highly intelligent, very well educated and extremely successful.... But he really knew absolutely nothing about my subject. In fact, one day he asked me, 'What is the difference between the solar system and the galaxy, and between the galaxy and the universe?' It was just a blur to him. So I thought, Okay, great, you're my audience. I find that extremely helpful, to have a person in mind. You can't write for the general public, but if there's somebody you're thinking of, I find that focuses the project."

Longitude and her next bestseller, Galileo's Daughter, both touch on astronomy. But Sobel fully indulges her love of the subject in The Planets. The book is a series of essays (Sobel thinks of them as short stories) on the heavenly bodies (including the Sun and the Moon) and has the potential to become, like Longitude, a classic of its kind.

The chapter on Jupiter opens with Galileo spying the giant planet's moons for the first time and naming them after his patron, Cosimo de' Medici, and his brothers. Galileo had cast the Florentine prince's horoscope—for, as Sobel writes, until that time, astronomy and astrology were inseparable. Sobel herself then relates the great astronomer's horoscope—which proves to be eerily accurate. As does Sobel's:

Sobel was born on June 15, 1947, which makes her a Gemini—the sign of the twins. Geminis are "mutable": they may have a range of interests and have trouble finding their true vocation.

"I had a really hard time in college because I couldn't make up my mind," Sobel says. "I changed my major five times. I changed schools three times." Eventually, she settled on theater history. Like the lost Commodore Anson whom Sobel once wrote about and who in 1741 sailed west across the Pacific, then east, then west again in a dizzying search for Juan Fernandez Island off the coast of Chile, Sobel zigged and zagged until she found her way with Longitude.

Geminis are word people. They are gifted communicators and may have a talent for writing.

Dava Sobel burst on the scene like a supernova in 1995 with the publication of Longitude, which sold 150,000 copies in hardcover, establishing her as a major popular-science writer and her publisher, Walker & Co., as a publishing player. But Sobel's is one of those stories of "overnight" success that comes after 20 years of working with words: as a technical writer at IBM, a women's page reporter at the Binghamton Evening Press and a writer on psychology ("about which I knew way too little," she says) at the New York Times.

Geminis are always seeking new experiences.

While speaking of the recent discovery of a possible 10th planet, Sobel becomes the questioner. "If you could be a journalist in space, would you go?" she asks me. I demur, not being the adventurous type. Would she? I ask in return.

"I think now I would. I'm older, so I feel I would take the risk. The only thing I'm afraid of is that I would be space sick." She identifies with Longitude's hero, John Harrison, who never found his sea legs when he sailed to the Continent to test his prized chronometer.

And Sobel has become an adventurer, going to sea and traveling for weeks to remote parts of the world to view an event that never lasts longer than seven minutes, one of the most spectacular of heavenly events: the solar eclipse, in which the Moon comes directly between the Earth and the Sun. She describes it in The Planets as only someone with firsthand knowledge could: "Pearl and platinum-colored streamers of coronal gas surround the vanished Sun like a jagged halo. Long red ribbons of electrified hydrogen leap from behind the black Moon and dance in the shimmering corona."

Sobel's most recent trip for an eclipse, earlier this year, took her to the Galápagos and then to the middle of the Pacific on a ship that was made for cruising the islands but not fit for the open sea: It "had no stabilizers. We were rocking so, and nothing in the cabins was secured, not even the televisions. The closet came off the wall, flew across the room." The eclipse's totality lasted all of 37 seconds—but it was "thrilling." Her next journey will be to Libya in 2006.

Geminis appreciate beauty and have a taste for art, music and poetry.

One of the implicit themes of The Planets is the extent to which our relationship with the heavens is embedded in human culture, in language, myth, music and art. Sobel quotes Lord Tennyson and Blake on Venus; she notes Venus's appearance in van Gogh's Road with Cypress and Star and relates the story behind composer Gustav Holst's Opus 32, The Planets, Suite for Orchestra.

Geminis are unconventional in their thinking and drawn to iconoclasts.

Who was more of an iconoclast than Copernicus? He is the subject of Sobel's next work—which is not a book but a play. "I actually had this idea 30 years ago, when I first read Arthur Koestler's Sleepwalkers, which was about the history of astronomy, and Kepler was the one he loved. But he wrote about Copernicus." When Sobel read in Koestler's book that Copernicus almost didn't write his great book De Revolutionibus, which posited the heliocentric orbits of the planets, that only as an old man was he urged on by one of his students, who helped him write the book, she started thinking: "That must have been some conversation. That's a play. What did they say to each other?"

Coming back to the story now, three decades later, she says, "I went to see a real live playwright who lives in my town, really to say, 'Is this a play?' He said, 'Yes, this is a play, and don't tell anybody.' " But Sobel is willing to tempt fate. "So I'm telling you now, because, why not? I always talk about what I'm working on, because that's how you can find the people who can help you with it."

And here's breaking news: Sobel has just had an asteroid named for her. Now her name is truly inscribed in the heavens she so loves to write about.