“Galileo argued that with a good telescope one could measure the angular sizes of stars, and that the stars typically measured a few arc-seconds in diameter,” says Chris Graney at Jefferson Community College in Louisvile in good ol’ Kentucky.
That doesn’t sound right. We know today that stars appear as point sources of light, so what was Galileo talking about?
Graney says that it looks increasingly likely that Galileo had developed an ingenious technique for measuring the angular size of distant objects. This allowed him, for example, to see that Jupiter’s apparent diameter became smaller as the distance between Earth and Jupiter increased.
So how did Galileo measure the angular size of stars?
The answer according to Graney is that Galileo must have been unknowingly measuring the diffraction pattern created by stars, the so-called airy disc that makes them seem to have an apparent diameter.
This was how he arrived at absurdly close estimates for their distance. For example, he thought that the brightest stars were about 360 astronomical units away, about an order of magnitude nearer than we think today.
Makes sense, I suppose. And since the wave mechanics necessary to understand diffraction wasn’t developed for two centuries after Galileo’s death, perhaps we should forgive him this one mistake.
Ref: arxiv.org/abs/0808.3411: Objects in the Telescope are Further Than They Appear
the physics arXiv blog 
August 28, 2008 at 3:08 am |
I disagree with the interpretation given in this preprint.
Tycho Brahe was also measuring the radius of stars and he never used any telescope. As a consequence, diffraction by the aperture of a telescope cannot explain that Galileo, like Tycho Brahe, was not seeing stars like point sources of light. A more plausible explanation is that light is diffused both in the atmosphere and in the medium inside the eye. Nothing to do with diffraction.
August 28, 2008 at 3:18 am |
Some closest red giants appear like finite diameter disk in telescopes. The angular diameter of Betelgeuse was first measured in 1920–1921 by Michelson. The entire Betelgeuse image fit entirely within a 10×10 pixel area on the Hubble Space Telescopes Faint Object Camera.
August 30, 2008 at 8:33 pm |
“This was how he arrived at absurdly close estimates for their distance. For example, he thought that the brightest stars were about 360 astronomical units away, about an order of magnitude nearer than we think today.”
Distance to Alpha Centauri = 4.3 L.Y. = 270,000 A.U.
So, he would have been off by **three** orders of magnitude.