[nphard85]

Google sent me to: https://www.scientificamerican.com/article/the-case-against-...

"The most devastating argument against the Copernican universe was the star size problem. When we look at a star in the sky, it appears to have a small, fixed width. Knowing this width and the distance to the star, simple geometry reveals how big the star is (right). In geocentric models of the universe, the stars lie just beyond the planets, implying that star sizes are comparable to that of the sun (below). But Copernicus's heliocentric theory demands that the stars be extremely far away. This in turn implies that they should be absurdly large—hundreds of times bigger than the sun (bottom). Copernicans could not explain away the anomalous data without appeals to divine intervention. In reality, the stars are far away, but their apparent width is an illusion, an artifact of the way light behaves as it enters a pupil or telescope—behavior that scientists would not understand for another 200 years."

[anovikov]

Ah so the Airy disk was confused for the star size? Then true; but that is sick, one had to just observe same star with the same telescope, once with full aperture and once with the aperture diaphragmed to half diameter, to observe that the star has become 2x larger (and 4x dimmer).

Also let's see, so a typical telescope aperture of the era have been 25-30mm (bigger will make focal ratio too big and chromatism totally unacceptable), which means Airy disk 5" big, or 1/40000 of the distance. Assuming stars being 1/15 parsec away to make their parallax just barely unobservable, it means yeah, stars 25x the size of Sun, which is whole lot and isn't a reasonable assumption.

But measuring Airy disk size is easy by using a hair put in the focal plane to see how quickly it crosses (will be 1/2 vs 1/4 of a second)... Easy experiment to check that Airy disk isn't a "star".

[fsh]

Diffraction of light was only understood in the mid 19th century. They simply had no idea what to look for. In Galilei's time, making optical instruments was mostly based on trial and error.

[anovikov]

OK one easier way to confirm that Airy disk isn't a "star". Speed of Moon movement vs stars was well known and is around 0.5'' per second, and Airy disk was 4-5'' wide. So, when a star is occulted by Moon, it should have disappeared slowly as the Moon "slides" over it, during ~10 seconds. In fact, it disappears instantly, because Airy disk isn't a star...

[whatshisface]

They wouldn't have needed to know what to look for if they had noticed that the "width" they ascribed to the stars changed with every measurement. Every single telescope size would have led them to a different figure, and if they had cared about pursuing quantitative inconsistencies they would have not only realized that they weren't measuring the width of the star, but also that the determining factor was the telescope's aperture.

[jacobush]

Came to here to say thank you for this comment, this made me understand what everyone was talking about. I know some photography so your comment made my mind "click" and understand. And I also agree with the comments the same level as this - they didn't understand these things back then. I couldn't at first understand why they'd believe the stars must have been huge.

[chopin]

Good reply but you have to know that you need to look for this. This is only obvious in hindsight.