By Katharine Sanderson

Galileo Galilei was right: Earth moves around the Sun, just as Nicolaus Copernicus said it did in 1543. But had Galileo followed the results of his observations to their logical conclusion, he should have backed another system -- the Tychonic view that Earth didn't move, and that everything else circled around it and the Sun, as developed by Danish astronomer Tycho Brahe in the sixteenth century.

This is the conclusion that Christopher Graney, a physicist at Jefferson Community and Technical College in Louisville, Kentucky, came to after reading manuscripts from another astronomer who was active in the late sixteenth and early seventeenth century, at the same time as Galileo.

Graney suggested in 2008 that Galileo's observations of stars were actually diffraction patterns called Airy disks -- patterns of concentric circles that arise when light from a point source, such as a star, passes through a hole. Diffraction hadn't been discovered in Galileo's time, so he was unaware of the phenomenon and believed what his eyes, or his telescope, were telling him and used the observations to estimate the size and distance of stars. As a result, he got the distances of the stars too short by a factor of thousands (see 'Galileo duped by diffraction').

After Graney realized that Airy disks had tricked Galileo, he decided to search for contemporaries of Galileo who might have seen similar things with their instruments. "There had to be someone who had a good telescope other than Galileo," says Graney.

That someone was German astronomer Simon Marius, most famous for naming the moons of Jupiter (Io, Europa, Ganymede and Callisto) and claiming to have detected them just days before Galileo.

Like Galileo, Marius mistook Airy disks as representing the stars themselves, says Graney in a paper soon to be published in the journal Physics in Perspective1.

Whereas Galileo stuck to his Copernican system view, Marius's analysis of starry data led him to very different conclusions, says Graney, who made the finding after reading a German translation of Marius's book Mundus Iovialis (The Jovian World), published in 1614.

Close call

According to Graney, Marius concluded that his observations showed that the stars were too close to Earth to satisfy the Copernican world view -- which says that the stars lie at a huge distance from Earth, and so would appear as starry pinpricks to any observer. The Copernican view was shared by others: stars would be seen as points if the telescope's lens was darkened by smoke, wrote Dutch astronomer Christiaan Huygens in his book Systema saturnium, published in 1659, 17 years after Galileo's death.

Instead, Marius said that the observation of the stars as disks confirmed the Tychonic system, which put Earth, unmoving, at the centre of the system with the Moon and Sun orbiting it. The planets Mercury, Venus, Mars, Jupiter and Saturn all then orbit the Sun and the stars lie just beyond these planets in a fixed sphere.

"Marius's reasoning was more rigorous than Galileo's," says Graney. "In fact, Galileo's own data would lead to the same conclusion, had he followed it rigorously." So why did Galileo stick to his Copernican views?

"Galileo was strongly committed to Copernicanism. That he chose not to include arguments against it is not very surprising, although according to modern scientific standards he probably should have done so," says Rienk Vermij, a historian of science from the University of Oklahoma in Norman. Vermij adds that the different world views were hotly debated for many years, and that this argument about the size and distribution of the stars was only one among many. "It is not evident that this argument should be decisive, any more than other arguments," says Vermij.

Graney can't say why Galileo stuck to what turned out to be the right view, in spite of the observations. "Galileo was a very smart guy. I wonder if he didn't have more of this worked out in his head that he never got around to putting down on paper," he says.

But in a world in which, according to Vermij, the Tychonic system was regarded as a serious rival of the Copernican system, Marius's conclusions seem reasonable. "You have to hand it to Simon Marius for looking at the data and pursuing it through to its logical conclusion," says Graney.