Published online 5 March 2010 | Nature |
doi:10.1038/news.2010.105 News Galileo backed Copernicus
despite data Stars viewed through early telescopes
suggested that Earth stood still. |
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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. ·
References |
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Did Galileo Galilei go with his gut rather than his
data? Justus Sustermans
(1636) |
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In the Copernican model, the stars appear as pinpricks
to observers on Earth. Stefano
Bianchetti/CORBIS |
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Simon Marius realized that his observations were
consistent with the Tychonic system. |
http://www.nature.com/news/2010/100305/full/news.2010.105.html