How long have we known Earth is round? A LONG TIME! There's a misconception that Christopher Columbus discovered Earth was round in 1492. No, not even close. It was common knowledge at that time Earth was round. Even Aristotle, around 2400 years ago, knew the Earth was round based on the shape of shadows during lunar eclipses. It's quite likely people knew Earth was round hundreds, if not thousands of years before Aristotle!
Recently several members of the anti-science party in the United States (you know which one this is) used the 'flat-Earth' argument to claim people are wrong on climate change. This was a group of climate denying politicians using the argument that people once thought Earth was flat and they were wrong. They then say that because these people were wrong, climate change scientists are wrong too. It's okay, you can smack your head on the wall repeatedly.
Um...yes, maybe thousands of years ago people once thought Earth was flat because they had no evidence to suggest otherwise, but long ago evidence was obtained to prove Earth was round. This is a horrible argument to use to deny climate change. The key is evidence. This is an amazing short-sighted view by these politicians that actually gets the entire argument wrong. People may have once thought Earth was flat due to lack of evidence. Then evidence showed us Earth is round. We once had no evidence to support a changing climate. Then, starting several decades ago we obtained stronger and stronger evidence Earth's climate is warming. The truth is always on the side of evidence. The argument these politicians are using actually argues FOR a flat Earth. It argues for the no evidence side. Sigh.