Published online 2 September 2008 | Nature |
doi:10.1038/news.2008.1073 News Galileo duped by diffraction Telescope pioneer foiled by optical
effect while measuring distance to the stars. |
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When Galileo Galilei used a new invention
called the telescope to watch the heavens, he revolutionized astronomy. But
his estimates of the distances to the stars were thousand of times too short. A scientist has now taken a closer look
at Galileo's seventeenth century results in an attempt to explain why the
estimates were so far off the mark1. Christopher Graney, a physicist at
Jefferson Community College in Louisville, Kentucky, argues in a paper posted
to the preprint server arXiv that Galileo was tricked by a phenomenon that
was only really understood two centuries later — diffraction. Graney says that Galileo was actually
observing the diffraction pattern that the stars created in the telescope,
instead of the stars themselves. Known as an Airy pattern, it arises when
light from a point source such as a star passes through a hole. The pattern
is made of concentric circles, with a bright 'Airy disk' in the middle —
which it seems that Galileo thought was the star. For fainter objects, the edges of the
disk are hard to see, making it look smaller, whereas brighter stars produce
a larger Airy disk. Because Galileo thought that all stars were the same size
and brightness as the Sun, he concluded that the smaller stars he observed
through his telescope were simply further away. So Galileo tried to infer the stars'
relative distance from Earth, in terms of what we now call astronomical units
(AU), by measuring their diameter. One astronomical unit is the distance from
Earth to the Sun, about 150 million kilometres. He deduced that the stars
were hundreds to thousands of AU away. In reality, the nearest stars are
about 300,000 AU away. Duplicitous disk To unravel Galileo's mistake, Graney
calculated the intensity of the diffraction pattern for stars of different
brightnesses. He then worked out Galileo's detection threshold and calculated
the size of the Airy disk that each different star would have produced in
Galileo's telescope. Drawing a graph of the stars' brightness
against the apparent diameter of the Airy disk gave Graney a roughly straight
line that looked very much like Galileo's own data — strong evidence, says
Graney, that the astronomer was indeed being fooled by the Airy disk. Historians have long known that Galileo
was looking at spurious images of the stars. But Graney's work pinpoints
exactly how diffraction could have tricked Galileo, says Noel Swerdlow, a
historian of science at the University of Chicago, Illinois. "Showing
the linear relationship between magnitude and apparent size does explain how
Galileo could believe that," he says. Although Galileo's assumptions about all
stars being identical to the Sun turned out to be wrong, they were reasonable
given the state of scientific knowledge in the seventeenth century, says
Graney. "He would have seen nothing to contradict that point of
view." Parallax view Astronomers now measure the distance to
stars using the parallax technique, in which the apparent location of a
distant star changes slightly as Earth orbits the Sun, allowing a distance to
be deduced from the angle between those locations. This technique was first
used in 1838 by German astronomer Friedrich Bessel. Graney's work shows just how good Galileo
was at taking measurements, says Don Salisbury, a physicist who teaches
history of science courses focusing on Galileo at Austin College, Texas.
"Galileo was indeed able to measure to an accuracy in which the
diffracted image would be measurable," he says. And Galileo's estimates were far larger
than the distances to any astronomical bodies known at the time. "300 AU
is close compared to modern ideas about the stars, but it is more than 10
times further than Neptune, and 30 times further than Saturn, the most
distant planet known in Galileo's day," says Graney. "It's a long
way, and I'm sure it seemed quite far to Galileo and his
contemporaries." ·
References 1.
Graney,
C. M. Preprint at http://arxiv.org/abs/0808.3411 (2008). |
Galileo
Galilei, as painted by Justus Sustermans in 1636. |
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