Sunspots, vortices of gas associated with strong electromagnetic activity, are visible as dark spots on the surface of the Sun but have never been sighted on the Sun's poles or equator.
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Are visible and have never been are not parallel....
Can you explain a more in detail the although??
Hi there! Actually, "are visible... but never have been..." are in parallel. They're not in the same tense, but they are parallel. A non-parallel version of this would be something like "are visible, ... and never having been ..."
With "although," we would need to restate the subject:
"appear... , although they have never been sighted..."
Please see this blog post for more about "although" clauses:
2 Explanations