[Grammar] Multiple tenses in a sentence.

[Eckaslike]

I think you could use either tense in the Magellan sentence.

I agree, perhaps it's a BrE usage because of what is implied by each sentence:

"Megellan proved that the earth was round [in 1521]."

"Magellan proved that the earth is round [because it is]."

 

[MikeNewYork]

I prefer "is" in that sentence. The earth is round (well, close anyway).