“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