TOEIC・English Undergraduate almost 5 yearsago

線を引いたところの訳出の仕方がよくわかりません

19 過去完了を見たら「基準となる時」を探せ 次の英文を訳しなさい Although the world was known by educated people to be 。 sphere, nobody had been all the way around it, and in Columbus's ime nobody knew how big it was. コを (玉川大)