Why were ancient people unable to detect stellar parallax?

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they couldn't measure small angles
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Why are stellar parallaxes difficult to measure from the ground?

From the ground, the atmosphere makes stars twinkle. This is called seeing. The breeze also can shake the telescope. At the observatory sites seeing

What is the stellar parallax for a star that is 20 PC away from Earth?

The stellar parallax for a star at 20PC distance is 1 / 20 of a second of angle. That's not much of an angle. It's the apparent width of a dimethat's about 46 miles away from

How do astronomers use parallax to determine stellar distances?

Parallax is the apparent shift (i.e. left to right) of a "foreground" object against the background when viewed from two different points. If you close one eye and view your f