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I am curious about the methods used by these early astronomers. TFA mentions Chinese astronomers observing sunspots in the third century BCE. How? Did they use a camera obscura, too? It also casually mentions "reconstructions based on tree rings." What? How do tree rings tell us about sunspot activity cycles?

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What? How do tree rings tell us about sunspot activity cycles?

IIRC, solar activity affects the level of charged particles hitting the upper atmosphere which in turn, affects the ratios of radioactive elements in the atmosphere. Traces of these variations can be found in the wood of trees that grew at the time. The tree rings provides dates for the growth.

Are there enough trees from 300 BCE that still survive today?

In the ones that do, wouldn't there be enough thermodynamic diffusion within the tree trunk to obfuscate the true isotope levels in each ring?

It also mentions telescope observations, did those go to a camera obscura after going through the telescope (is that possible?) or were people actually just staring at the sun through a telescope? How did they not go blind?

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