July 22, 2009.

If you're into astronomy, you know what that date means: one heck of a total solar eclipse. And you know where you want to be: China or the South Pacific.

Just don't wait long to book a trip.

Tours to see what could be one of the most spectacular total solar eclipses in years are selling out almost as soon as they're announced.

"Our tour was filling up so fast, we had to create a second departure," says Victoria Sahami, owner of Sirius Travel, a company that specializes in eclipse tours. "Now we're thinking of adding a third."

Eclipses happen on average about every 18 months. But not all are created equal. Some last just a few seconds; others darken the sky for up to seven minutes. And where they occur makes a huge difference.

"Sometimes an eclipse falls in the ocean, and the only way to see it is to charter a ship," Sahami says.

Others take place in areas where frequent cloud cover, humidity or haze make for poor viewing.

The 2009 eclipse will occur over relatively easy-to-reach areas of China where the weather should cooperate. But what's really getting skygazers excited is the predicted length: more than five minutes of "totality" if you watch from the right spot in China and even longer if you can get out to sea near Iwo Jima. That's an eternity to an eclipse junkie, Sahami says. By comparison, she says, Sirius clients traveled thousands of miles to Australia in 2002 to see an eclipse that lasted just 29 seconds.

More than half a dozen tour companies have announced trips for the event. (Many also are selling trips to a shorter, harder-to-reach eclipse occurring over Mongolia on Aug. 1 of the next year.) The typical tour is accompanied by an astronomer and takes in astronomical sites along the way as well as more traditional attractions.

Among them: