Galileo's telescope, here in Philadelphia

When NASA launched a new space telescope called Kepler this year, mankind took another step in a quest that started 400 years ago with two eyeglass lenses and a piece of lead pipe.

It was in 1609 that mathematics professor Galileo Galilei pointed his homemade telescope skyward and saw what looked like mountains on the moon and other wonders no one had imagined.

His instrument - marginally more powerful than a cheap pair of modern binoculars - enabled him to shatter cosmological dogma as he carefully catalogued the phases of Venus, the moons of Jupiter, and the stars of the Milky Way.

Starting Saturday, visitors to the Franklin Institute in Philadelphia can see one of two surviving original telescopes that allowed Galileo to open the heavens to science - and ultimately led to his house arrest for heresy.

Galileo's revolution continues today, as ever more powerful telescopes keep demoting Earth farther from the center of all creation. The Kepler telescope promises to show whether the universe is sprinkled with other earths.

Historians say Galileo was particularly worried about rivals, especially the formidable astronomer Johannes Kepler, for whom the new NASA telescope is named.

Galileo feared Kepler would get his hands on a telescope and beat him to the cosmic discoveries that awaited. But Kepler apparently lacked Galileo's gift for building instruments.

It was in the summer of 1609 that Galileo, then teaching at the University of Padua, heard about a new magnifying invention that used two lenses and a tube.

These early telescopes magnified by only a factor of three, said Owen Gingerich, a Harvard historian of science. Galileo didn't wait to acquire one. With the concept in his head, he starting building much better ones, gradually working from about six power magnification to 20.

"Galileo turned what was essentially a carnival toy into a serious scientific instrument," Gingerich said.

He worked fast, and by fall he had a telescope with enough power to resolve topography on the moon - what we now know as craters.

Back in the early 17th century, respectable, educated Europeans were taught that Earth was the center of the universe, and that the stars, planets, moon, and sun were perfectly spherical and made of a heavenly substance called ether. They rotated around Earth on crystalline shells, according to the book Galileo's New Universe.

The more Galileo saw, the more strongly he argued for a cosmos in which Earth and other planets moved around the sun, with the stars much farther away. It was a view that had been suggested more that 50 years earlier in a book by the astronomer Nicholas Copernicus.

It was not just religious dogma that kept the idea from wide acceptance. It was also common intuition, said Gingerich. Earth certainly didn't feel as if it were moving. And people argued that there was no plausible way for the moon to ride around the sun along with the planet.

But Galileo showed that other planets could have their own satellites when he discovered Jupiter's moons and carefully logged their orbits.

If Jupiter could move through the heavens with four moons in tow, he argued, Earth could move with its one.

Galileo also made a case that the stars were much farther away than the planets - and that they were self-illuminating rather than reflecting sunlight.

No one knows what Galileo thought about the prospect of other inhabited worlds. His silence on that issue may be related to the death of philosopher and teacher Giordano Bruno, who was burned at the stake in 1600 for heresy.

Bruno speculated that the universe was populated by infinite worlds circling infinite suns. (He made many other inflammatory remarks such as the claim that Jesus was not divine but simply a good magician.)

While his visionary picture of the universe wasn't the central reason for Bruno's execution, Gingerich said, people didn't talk too loudly about other worlds for some years.

The battle between science and religion in Galileo's day wasn't all black and white either, Gingerich said.

Galileo provoked church authorities by stating that people should look to the Bible to tell them how to go to heaven but not to tell them how the heavens go. He stayed quiet for years, but in 1624, with a new pope in power, Galileo decided to record his observations and ideas in a book.

"Everyone expected he would write a dry textbook," Gingerich said. Most likely he would use Latin, the language of scholars. But Galileo wrote in the vernacular Italian and created a rousing dialogue among three characters. There was a wise judge, a character representing Galileo, and another he dubbed "Simplicio," implying that a simpleton was arguing the church's position.

After that he was put under house arrest for the remainder of his life.

Four hundred years later, the sun has lost its central position along with Earth and is now seen as swirling around on the outskirts of a vast galaxy of stars, itself one of billions of other galaxies.

Popular wisdom holds that some of those billions of stars must harbor planets like Earth.

It wasn't until 1995 that someone detected such a planet - not with a direct image, but by measuring how the gravitational pull of a large planet causes a star's orbit to wobble.

From that wobble, astronomers deduced that the planet was about the size of Jupiter and hellishly close to its sun.

Since then, astronomers have found more than 300 other planets, most of them so-called gas giants like Jupiter. More recently, they have started finding "super-earths" about five times the size of Earth.

But there's no way to see the effects of a planet as tiny as Earth from the ground. The "noise" from the atmosphere would drown it out.

So NASA this year launched the Kepler telescope into orbit, where it will stare in one direction to look for planets among 150,000 stars.

Kepler can't see the planets but can detect them when they cross in front of their stars, said astronomer Jon Jenkins of the SETI Institute in California, which searches for extraterrestrial life. Those crossings dim the stars just a tiny bit.

So many big Jupiter-size planets have been found that scientists estimate they orbit about a fifth of all stars, said astronomer Alan Boss of the Carnegie Institution and author of The Crowded Universe. In theory, smaller planets should be easier to form, he said, so they could be circling around the majority of stars.

Looking ahead, scientists have drawn up blueprints for yet more ambitious space-based telescopes that could take pictures of these planets, if they exist, and tease out their compositions.

Methane, oxygen, or ozone would hint that something is alive up there.

"I'm sure Kepler will find things nobody imagined," SETI's Jenkins said.

Just as nobody imagined 400 years ago that Galileo would see craters on the moon.

See more at the Franklin Institute's Web site via http://go.philly.com/galileoEndText